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  • How to check if internet connection is present in java?

    - by Chris
    How do you check if you can connect to the internet via java? One way would be: final URL url = new URL("http://www.google.com"); final URLConnection conn = url.openConnection(); ... if we got here, we should have net ... But is there something more appropriate to perform that task, especially if you need to do consecutive checks very often and a loss of internet connection is highly probable?

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  • disk-to-disk backup without costly backup redundancy?

    - by AaronLS
    A good backup strategy involves a combination of 1) disconnected backups/snapshots that will not be affected by bugs, viruses, and/or security breaches 2) geographically distributed backups to protect against local disasters 3) testing backups to ensure that they can be restored as needed Generally I take an onsite backup daily, and an offsite backup weekly, and do test restores periodically. In the rare circumstance that I need to restore files, I do some from the local backup. Should a catastrophic event destroy the servers and local backups, then the offsite weekly tape backup would be used to restore the files. I don't need multiple offsite backups with redundancy. I ALREADY HAVE REDUNDANCY THROUGH THE USE OF BOTH LOCAL AND REMOTE BACKUPS. I have recovery blocks and par files with the backups, so I already have protection against a small percentage of corrupt bits. I perform test restores to ensure the backups function properly. Should the remote backups experience a dataloss, I can replace them with one of the local backups. There are historical offsite backups as well, so if a dataloss was not noticed for a few weeks(such as a bug/security breach/virus), the data could be restored from an older backup. By doing this, the only scenario that poses a risk to complete data loss would be one where both the local, remote, and servers all experienced a data loss in the same time period. I'm willing to risk that happening since the odds of that trifecta negligibly small, and the data isn't THAT valuable to me. So I hope I have emphasized that I don't need redundancy in my offsite backups because I have covered all the bases. I know this exact technique is employed by numerous businesses. Of course there are some that take multiple offsite backups, because the data is so incredibly valuable that they don't even want to risk that trifecta disaster, but in the majority of cases the trifecta disaster is an accepted risk. I HAD TO COVER ALL THIS BECAUSE SOME PEOPLE DON'T READ!!! I think I have justified my backup strategy and the majority of businesses who use offsite tape backups do not have any additional redundancy beyond what is mentioned above(recovery blocks, par files, historical snapshots). Now I would like to eliminate the use of tapes for offsite backups, and instead use a backup service. Most however are extremely costly for $/gb/month storage. I don't mind paying for transfer bandwidth, but the cost of storage is way to high. All of them advertise that they maintain backups of the data, and I imagine they use RAID as well. Obviously if you were using them to host servers this would all be necessary, but for my scenario, I am simply replacing my offsite backups with such a service. So there is no need for RAID, and absolutely no value in another layer of backups of backups. My one and only question: "Are there online data-storage/backup services that do not use redundancy or offer backups(backups of my backups) as part of their packages, and thus are more reasonably priced?" NOT my question: "Is this a flawed strategy?" I don't care if you think this is a good strategy or not. I know it pretty standard. Very few people make an extra copy of their offsite backups. They already have local backups that they can use to replace the remote backups if something catastrophic happens at the remote site. Please limit your responses to the question posed. Sorry if I seem a little abrasive, but I had some trolls in my last post who didn't read my requirements nor my question, and were trying to go off answering a totally different question. I made it pretty clear, but didn't try to justify my strategy, because I didn't ask about whether my strategy was justifyable. So I apologize if this was lengthy, as it really didn't need to be, but since there are so many trolls here who try to sidetrack questions by responding without addressing the question at hand.

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  • How do I achieve lossless JPEG joining without truncation of partial MCUs?

    - by Karan
    I am working on a project for which I need to join thousands of JPEG images losslessly (I'm not talking about the Lossless JPEG/JPEG 2000/JPEG-LS formats here). Aforementioned images have varying levels of chroma subsampling (1x1, 1x2, 2x1, 2x2), resulting in varying MCU sizes (8x8, 8x16, 16x8, 16x16 px). However, in any given set of images to be joined together, each image has identical characteristics. For now, let's assume I only have 2 images. Image #1 (I1) is 256x256px in size and #2 (I2) is 239x256px in size. 2x2 subsampling is used such that MCU size is 16x16px. I2 thus obviously has partial MCUs at the right edge, since its width is not evenly divisible by 16. (I've read that so-called 'partial' MCUs actually contain the data for a complete MCU, but the image dimensions instruct the renderer to only display the relevant pixels and ignore/hide the extra ones.) Looking around for tools that could help me accomplish this, I came across a modified version of JpegTran, that contains an experimental lossless crop 'n' drop (cut & paste) feature. All the other apps I encountered that support lossless JPEG editing seem to utilise IJG's (JpegTran) code, so this seemed to be the logical choice. Also, given the sheer number of images, I wanted something that could preferably be run from the command-line so that I could automate the process with a script. Unfortunately, while everything else worked fine, it seems JpegTran truncates the partial MCUs instead of retaining them. Thus in the example above, the final joined image contains all of I1, but only 224x256px of I2. Why 224? because 239 = 14x16+15, which means there are 14 full MCUs along the width, and 1 partial MCU (just 1px short of the complete 16px). The last 15px is what is getting blanked, leading to a 495x256px image with 15px of blank (grey) pixels at the right edge. See images below (shame that imgur re-compresses them): (left )+ (right) = As you can clearly see, the red portion (15px) of I2 has been truncated by JpegTran. If the MCUs were 8px in width, the lost portion would have been the right-most 7px of I2. Similarly, joining I3 (256x239px) *below * I1 would cause the loss of 7 or 15px, depending on the MCU height of course: (top) + (bottom) = If this is better suited to some other StackExchange (or even non-SE) site/forum where JPEG/image encoding experts hang out, do let me know. Can what I am attempting even be done, or is the so-called 'lossless' JPEG crop 'n' drop only valid for images with no partial MCUs? (Maybe that is why the feature is still in an "experimental state" more than a decade after being introduced...) Until I know for sure that it is impossible, I am not interested in suggestions for lossy joining. Avoiding any generational loss whatsoever is the sole reason why I'm breaking my head over this, else I'd have had this done and dusted ages ago. Also, I am not interested in suggestions related to switching image formats. I do not control the source of the images. If it can be done, how? Please keep in mind that any alternate apps suggested must ideally be capable of automation, given the requirements stated above. (But given how it's unlikely I'm even going to receive a useful answer given the constraints, I would be happy with any app suggestion just as long as it actually works. I can always look into an AutoIT/AHK script or something later to automate it.) I understand that an odd-sized final image might cause issues, so I am fully prepared to accept any solution, even if it results in blank (preferably black) padding pixels to the right/bottom. What I mean is, I don't care if I1 + I2 is 496x256px (1px padding) or even 512x256px (17px padding) in size, as long as the final image contains all the actual image data from both source images, and the entire process is lossless. Obviously the lesser the padding (if any), the better, but at this point any solution will do. A Windows-based solution would be perfect, but a Linux-based one would be entirely acceptable.

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  • Cannot ping host stale ARP cache?

    - by gkchicago
    I am having a strange issue with a Debian (Lenny/Linux 2.6.26-2-amd64) that has been driving me nuts. On some machines within my network I can ping the host in question just fine, other times I have to manually hard-code the ARP ethernet address for the IP in order to establish connectivity. I've finally worked it down to somehow involving ARP. I just found how to fix it in a way that made it work but I'm looking for help explaining this issue and also I don't trust my fix to be permanent.. My thought process has been the following but I just can't make any sense out of it: Could it be the card? (Intel 82555 rev 4) Could it be because there are two network cards? (Default route is eth0) Could it be because of the network aliases? Lenny? AMD x86_64? Argh.. Thank you for any insight you might have // Ping doesn't go thru [gordon@ubuntu ~]$ ping 192.168.135.101 PING 192.168.135.101 (192.168.135.101) 56(84) bytes of data. --- 192.168.135.101 ping statistics --- 4 packets transmitted, 0 received, 100% packet loss, time 3014ms // Here's the ARP Table, sometimes the .151 address is good, sometimes it // also matches the Gateways MAC like .101 is doing right here. [gordon@ubuntu ~]$ cat /proc/net/arp IP address HW type Flags HW address Mask Device 192.168.135.15 0x1 0x2 00:0B:DB:2B:24:89 * eth0 192.168.135.151 0x1 0x2 00:0B:6A:3A:30:A6 * eth0 192.168.135.1 0x1 0x2 00:1A:A2:2D:2A:04 * eth0 192.168.135.101 0x1 0x2 00:1A:A2:2D:2A:04 * eth0 // Drop the bad arp table listing and set it manually based on /sbin/ifconfig [gordon@ubuntu ~]$ sudo arp -d 192.168.135.101 [gordon@ubuntu ~]$ sudo arp -s 192.168.135.101 00:0B:6A:3A:30:A6 // Ping starts going thru..?!? [gordon@ubuntu ~]$ ping 192.168.135.101 PING 192.168.135.101 (192.168.135.101) 56(84) bytes of data. 64 bytes from 192.168.135.101: icmp_seq=1 ttl=64 time=15.8 ms 64 bytes from 192.168.135.101: icmp_seq=2 ttl=64 time=15.9 ms 64 bytes from 192.168.135.101: icmp_seq=3 ttl=64 time=16.0 ms 64 bytes from 192.168.135.101: icmp_seq=4 ttl=64 time=15.9 ms --- 192.168.135.101 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3012ms rtt min/avg/max/mdev = 15.836/15.943/16.064/0.121 ms The following is my network config on this. gordon@db01:~$ /sbin/ifconfig eth0 Link encap:Ethernet HWaddr 00:0b:6a:3a:30:a6 inet addr:192.168.135.151 Bcast:192.168.135.255 Mask:255.255.255.0 inet6 addr: fe80::20b:6aff:fe3a:30a6/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:15476725 errors:0 dropped:0 overruns:0 frame:0 TX packets:10030036 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:18565307359 (17.2 GiB) TX bytes:3412098075 (3.1 GiB) eth0:0 Link encap:Ethernet HWaddr 00:0b:6a:3a:30:a6 inet addr:192.168.135.150 Bcast:192.168.135.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 eth0:1 Link encap:Ethernet HWaddr 00:0b:6a:3a:30:a6 inet addr:192.168.135.101 Bcast:192.168.135.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 eth1 Link encap:Ethernet HWaddr 00:e0:81:2a:6e:d0 inet addr:10.10.62.1 Bcast:10.10.62.255 Mask:255.255.255.0 inet6 addr: fe80::2e0:81ff:fe2a:6ed0/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:10233315 errors:0 dropped:0 overruns:0 frame:0 TX packets:19400286 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:1112500658 (1.0 GiB) TX bytes:27952809020 (26.0 GiB) Interrupt:24 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:387 errors:0 dropped:0 overruns:0 frame:0 TX packets:387 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:41314 (40.3 KiB) TX bytes:41314 (40.3 KiB) gordon@db01:~$ sudo mii-tool -v eth0 eth0: negotiated 100baseTx-FD, link ok product info: Intel 82555 rev 4 basic mode: autonegotiation enabled basic status: autonegotiation complete, link ok capabilities: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD advertising: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control link partner: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD gordon@db01:~$ sudo route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface localnet * 255.255.255.0 U 0 0 0 eth0 10.10.62.0 * 255.255.255.0 U 0 0 0 eth1 default 192.168.135.1 0.0.0.0 UG 0 0 0 eth0

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  • how to make bridge networking with KVM work in Fedora19

    - by netllama
    I'm attempting to get several virtual machines setup on a Fedora-19 host system, with the traditional bridge network devices (br0, br1, etc). I've done this many times before with older versions of Fedora (16, 14, etc), and it just works. However, for reasons that I cannot figure out, the bridge doesn't seem to be working in Fedora19. While I can successfully connect to the outside world (local network + internet) from inside a VM, nothing can communicate with the VM from outside (local network). I'm referring to something as trivial as pinging. From inside the VM, I can ping anything successfully (0% packet loss). However, from outside the VM (on the host, or any other system on the same network), I see 100% packet loss when pinging the IP address of the VM. My first question is simply, does anyone else have this working successfully in F19? And if so, what steps did you need to follow? I'm not using NetworkManager at all, its all the network service. There are no firewalls involved anywhere (iptables & firewall services are currently disabled). Here's the current host configuration: # brctl show bridge name bridge id STP enabled interfaces br0 8000.38eaa792efe5 no em2 vnet1 br1 8000.38eaa792efe6 no em3 br2 8000.38eaa792efe7 no em4 vnet0 virbr0 8000.525400db3ebf yes virbr0-nic # more /etc/sysconfig/network-scripts/ifcfg-em2 TYPE=Ethernet BRIDGE="br0" NAME=em2 DEVICE="em2" UUID=aeaa839e-c89c-4d6e-9daa-79b6a1b919bd ONBOOT=yes HWADDR=38:EA:A7:92:EF:E5 NM_CONTROLLED="no" # more /etc/sysconfig/network-scripts/ifcfg-br0 TYPE=Bridge NM_CONTROLLED="no" BOOTPROTO=dhcp NAME=br0 DEVICE="br0" ONBOOT=yes # ifconfig em2 ;ifconfig br0 em2: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet6 fe80::3aea:a7ff:fe92:efe5 prefixlen 64 scopeid 0x20<link> ether 38:ea:a7:92:ef:e5 txqueuelen 1000 (Ethernet) RX packets 100093 bytes 52354831 (49.9 MiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 25321 bytes 15791341 (15.0 MiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 device memory 0xf7d00000-f7e00000 br0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 10.31.99.226 netmask 255.255.252.0 broadcast 10.31.99.255 inet6 fe80::3aea:a7ff:fe92:efe5 prefixlen 64 scopeid 0x20<link> ether 38:ea:a7:92:ef:e5 txqueuelen 0 (Ethernet) RX packets 19619 bytes 1963328 (1.8 MiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 11 bytes 1074 (1.0 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 Relevant section from /etc/libvirt/qemu/foo.xml (one of the VMs with this problem): <interface type='bridge'> <mac address='52:54:00:26:22:9d'/> <source bridge='br0'/> <model type='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/> </interface> # ps -ef | grep qemu qemu 1491 1 82 13:25 ? 00:42:09 /usr/bin/qemu-system-x86_64 -machine accel=kvm -name cuda-linux64-build5 -S -machine pc-0.13,accel=kvm,usb=off -cpu SandyBridge,+pdpe1gb,+osxsave,+dca,+pcid,+pdcm,+xtpr,+tm2,+est,+smx,+vmx,+ds_cpl,+monitor,+dtes64,+pbe,+tm,+ht,+ss,+acpi,+ds,+vme -m 16384 -smp 6,sockets=6,cores=1,threads=1 -uuid 6e930234-bdfd-044d-2787-22d4bbbe30b1 -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/cuda-linux64-build5.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=localtime -no-shutdown -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -drive file=/var/lib/libvirt/images/cuda-linux64-build5.img,if=none,id=drive-virtio-disk0,format=raw,cache=writeback -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -netdev tap,fd=25,id=hostnet0,vhost=on,vhostfd=26 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:26:22:9d,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -vnc 127.0.0.1:1 -vga cirrus -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5 I can provide additional information, if requested. thanks!

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  • Intermittent 404 on select assets, LAMP stack

    - by Tom Lagier
    We have a LAMP stack WordPress server that is serving most assets correctly. However, one plugin's CSS file and several images are returning soft 404s roughly 20% of the time. I can't find any reference to the 404 in the access logs, but the browser is definitely receiving a 404 response from somewhere (WordPress, I would assume). When I use an alias URL that does not match the site URL but does resolve to the asset path, the resource loads correctly 100% of the time. However, using the site url only resolves for the select, problematic assets 20% of the time. You can test one of the problematic assets here: http://www.mreco.org/wp-content/uploads/2014/05/zero-cost.jpg However the alias link always resolves correctly: http://mr-eco.wordpress.promocampaigns.com/wp-content/uploads/2014/05/zero-cost.jpg Stranger, if I attempt to access outdated content that definitely does not exist on the server, at the live URL it returns the content roughly 50% of the time. Using the alias link, it 404s 100% of the time - the correct behavior. Error log and PHP error log are clean. A sample access log (pulled from grep 'zero-cost.jpg' /var/log/httpd/mr-eco-access_log) from several refreshes of the live direct link (where I am not seeing any 404's): 10.166.202.202 - - [28/May/2014:20:27:41 +0000] "GET /wp-content/uploads/2014/05/zero-cost.jpg HTTP/1.1" 304 - 10.166.202.202 - - [28/May/2014:20:27:42 +0000] "GET /wp-content/uploads/2014/05/zero-cost.jpg HTTP/1.1" 304 - 10.166.202.202 - - [28/May/2014:20:27:43 +0000] "GET /wp-content/uploads/2014/05/zero-cost.jpg HTTP/1.1" 304 - 10.166.202.202 - - [28/May/2014:20:27:43 +0000] "GET /wp-content/uploads/2014/05/zero-cost.jpg HTTP/1.1" 304 - 10.176.201.37 - - [28/May/2014:20:27:56 +0000] "GET /wp-content/uploads/2014/05/zero-cost.jpg HTTP/1.1" 200 57027 Chrome's dev tools list the following network activity before displaying 404 page content: zero-cost.jpg /wp-content/uploads/2014/05 GET 404 Not Found text/html Other 15.9?KB 73.2?KB 953?ms 947?ms My Apache configuration is standard, I've listed the virtual host entry and .htaccess file below. I can provide other parts of Apache config if necessary. Virtual host: <VirtualHost *:80> DocumentRoot /var/www/public_html/mr-eco.wordpress.promocampaigns.com ServerName www.mreco.org ServerAlias mreco.org mr-eco.wordpress.promocampaigns.com ErrorLog logs/mr-eco-error_log CustomLog logs/mr-eco-access_log common <Directory /var/www/public_html/mr-eco.wordpress.promocampaigns.com> AllowOverride All SetOutputFilter DEFLATE </Directory> </VirtualHost> .htaccess: # BEGIN WordPress <IfModule mod_rewrite.c> RewriteEngine On RewriteBase / RewriteRule ^index\.php$ - [L] RewriteCond %{REQUEST_FILENAME} !-f RewriteCond %{REQUEST_FILENAME} !-d RewriteRule . /index.php [L] </IfModule> # END WordPress I have checked for multiple A records and can confirm that there is a single A record pointing at the domain: ;; ANSWER SECTION: mreco.org. 60 IN A 50.18.58.174 I'm fairly new to systems administration, and at a complete loss as to what could cause this. In the past, inconsistently 404ing assets have been because of out-of-sync instances behind a load balancer. In this case, it is a single instance behind the load balancer. Because of the inconsistency, it feels like a caching issue. We don't make use of Apache caching, and as far as I know WordPress should not be caching either. What I've done so far: Reset WordPress permalinks Disabled WordPress plugins Re-generated WordPress .htaccess file Swapped ServerName and ServerAlias directives Cleared browser cache Confirmed disk location of resources Checked PHP, access, and error logs Confirmed correct DNS setup (can post if necessary) I'm at a total loss. Thanks for helping me out!

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  • simple and reliable centralized logging inside Amazon VPC

    - by Nakedible
    I need to set up centralized logging for a set of servers (10-20) in an Amazon VPC. The logging should be as to not lose any log messages in case any single server goes offline - or in the case that an entire availability zone goes offline. It should also tolerate packet loss and other normal network conditions without losing or duplicating messages. It should store the messages durably, at the minimum on two different EBS volumes in two availability zones, but S3 is a good place as well. It should also be realtime so that the messages arrive within seconds of their generation to two different availability zones. I also need to sync logfiles not generated via syslog, so a syslog-only centralized logging solution would not fulfill all the needs, although I guess that limitation could be worked around. I have already reviewed a few solutions, and I will list them here: Flume to Flume to S3: I could set up two logservers as Flume hosts which would store log messages either locally or in S3, and configure all the servers with Flume to send all messages to both servers, using the end-to-end reliability options. That way the loss of a single server shouldn't cause lost messages and all messages would arrive in two availability zones in realtime. However, there would need to be some way to join the logs of the two servers, deduplicating all the messages delivered to both. This could be done by adding a unique id on the sending side to each message and then write some manual deduplication runs on the logfiles. I haven't found an easy solution to the duplication problem. Logstash to Logstash to ElasticSearch: I could install Logstash on the servers and have them deliver to a central server via AMQP, with the durability options turned on. However, for this to work I would need to use some of the clustering capable AMQP implementations, or fan out the deliver just as in the Flume case. AMQP seems to be a yet another moving part with several implementations and no real guidance on what works best this sort of setup. And I'm not entirely convinced that I could get actual end-to-end durability from logstash to elasticsearch, assuming crashing servers in between. The fan-out solutions run in to the deduplication problem again. The best solution that would seem to handle all the cases, would be Beetle, which seems to provide high availability and deduplication via a redis store. However, I haven't seen any guidance on how to set this up with Logstash and Redis is one more moving part again for something that shouldn't be terribly difficult. Logstash to ElasticSearch: I could run Logstash on all the servers, have all the filtering and processing rules in the servers themselves and just have them log directly to a removet ElasticSearch server. I think this should bring me reliable logging and I can use the ElasticSearch clustering features to share the database transparently. However, I am not sure if the setup actually survives Logstash restarts and intermittent network problems without duplicating messages in a failover case or similar. But this approach sounds pretty promising. rsync: I could just rsync all the relevant log files to two different servers. The reliability aspect should be perfect here, as the files should be identical to the source files after a sync is done. However, doing an rsync several times per second doesn't sound fun. Also, I need the logs to be untamperable after they have been sent, so the rsyncs would need to be in append-only mode. And log rotations mess things up unless I'm careful. rsyslog with RELP: I could set up rsyslog to send messages to two remote hosts via RELP and have a local queue to store the messages. There is the deduplication problem again, and RELP itself might also duplicate some messages. However, this would only handle the things that log via syslog. None of these solutions seem terribly good, and they have many unknowns still, so I am asking for more information here from people who have set up centralized reliable logging as to what are the best tools to achieve that goal.

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  • Win 7 Netbook refuses to ping JetDirect card (all other PCs work)

    - by Luke Puplett
    I have an odd thing occuring here. From a Windows 7 netbook, I cannot ping an HP printer on the network, while all other machines (Win7/Vista) can. And the netbook can also ping everything else on the LAN. Example showing that the netbook can ping 192.168.3.4 but not 3.6. C:\Users\backdoor>ping w7ue1m Pinging w7ue1m.corp.biz.co.uk [192.168.3.4] with 32 bytes of data: Reply from 192.168.3.4: bytes=32 time=7ms TTL=128 Reply from 192.168.3.4: bytes=32 time=4ms TTL=128 Reply from 192.168.3.4: bytes=32 time=2ms TTL=128 Reply from 192.168.3.4: bytes=32 time=2ms TTL=128 Ping statistics for 192.168.3.4: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 2ms, Maximum = 7ms, Average = 3ms C:\Users\backdoor>ping uktnprint1 Pinging uktnprint1.corp.biz.co.uk [192.168.3.6] with 32 bytes of data: Reply from 192.168.3.0: Destination host unreachable. Reply from 192.168.3.0: Destination host unreachable. Reply from 192.168.3.0: Destination host unreachable. Reply from 192.168.3.0: Destination host unreachable. Ping statistics for 192.168.3.6: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),`enter code here` The IPCONFIG result for the netbook is fine. IPv4 Address. . . . . . . . . . . : 192.168.3.0 Subnet Mask . . . . . . . . . . . : 255.255.0.0 Default Gateway . . . . . . . . . : 192.168.1.1 Most unusual network thing I've seen in years. I must reiterate that only this netbook is having trouble pinging/printing. Thanks, Luke ** UPDATE ** Am now on a Vista box, and here's the IPCONFIG: IPv4 Address. . . . . . . . . . . : 192.168.3.3 Subnet Mask . . . . . . . . . . . : 255.255.0.0 Default Gateway . . . . . . . . . : 192.168.1.1 Pinging uktnprint1.corp.biz.co.uk [192.168.3.6] with 32 bytes of data: Reply from 192.168.3.6: bytes=32 time=2ms TTL=60 Firewall is off. I'll look into the chance of an IP conflict because it's the only thing I can think of - compare arp caches of each machine. Cheers!

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  • Cisco 678 Will Not Work using PPPoE - Possibly Because I Configured it Incorrectly..?

    - by Brian Stinar
    I am attempting to configure a Cisco 678 because I am totally sick on my Actiontec. However, I am running into some problems. It seems as though the Cisco is able to train the line, but I am unable to ping out. I am all right at programming, but still learning a lot when it comes to being a system administrator. I apologize in advance if I did something ridiculous, or am attempting to configure this device to do something it was not designed to do. It is almost like I am not correctly configuring the device to grab it's IP using PPPoA (like my Actiontec.) The output from "show running" (below) makes me think this too. Below are the commands I ran in order to configure this: # en # set nvram erase # write # reboot # en # set nat enable # set dhcp server enable # set PPP wan0-0 ipcp 0.0.0.0 # set ppp wan0-0 dns 0.0.0.0 # set PPP wan0-0 login xxxxx // My actual login # set PPP wan0-0 password yyyyy // My actual password # set PPP restart enabled # set int wan0-0 close # set int wan0-0 vpi 0 # set int wan0-0 vci 32 # set int wan0-0 open # write # reboot Here is the output from a few commands I thought could provide some useful information: cbos#ping 74.125.224.113 Sending 1 8 byte ping(s) to 74.125.224.113 every 2 second(s) Request timed out cbos#show version Cisco Broadband Operating System CBOS (tm) 678 Software (C678-I-M), Version v2.4.9 - Release Software Copyright (c) 1986-2001 by cisco Systems, Inc. Compiled Nov 17 2004 15:26:29 DMT FULL firmware version G96 NVRAM image at 0x1030f000 cbos#show errors - Current Error Messages - ## Ticks Module Level Message 0 000:00:00:00 PPP Info IPCP Open Event on wan0-0 1 000:00:00:14 ATM Info Wan0 Up 2 000:00:00:14 PPP Info PPP Up Event on wan0-0 3 000:00:01:54 PPP Info PPP Down Event on wan0-0 Total Number of Error Messages: 4 cbos#show interface wan0 wan0 ADSL Physical Port Line Trained Actual Configuration: Overhead Framing: 3 Trellis Coding: Enabled Standard Compliance: T1.413 Downstream Data Rate: 1184 Kbps Upstream Data Rate: 928 Kbps Interleave S Downstream: 4 Interleave D Downstream: 16 Interleave R Downstream: 16 Interleave S Upstream: 4 Interleave D Upstream: 8 Interleave R Upstream: 16 Modem Microcode: G96 DSP version: 0 Operating State: Showtime/Data Mode Configured: Echo Cancellation: Disabled Overhead Framing: 3 Coding Gain: Auto TX Power Attenuation: 0dB Trellis Coding: Enabled Bit Swapping: Disabled Standard Compliance: T1.413 Remote Standard Compliance: T1.413 Tx Start Bin: 0x6 Tx End Bin: 0x1f Data Interface: Utopia L1 Status: Local SNR Margin: 19.0dB Local Coding Gain: 7.5dB Local Transmit Power: 12.5dB Local Attenuation: 46.0dB Remote Attenuation: 31.0dB Local Counters: Interleaved RS Corrected Bytes: 0 Interleaved Symbols with CRC Errors: 2 No Cell Delineation Interleaved: 0 Out of Cell Delineation Interleaved: 0 Header Error Check Counter Interleaved: 0 Count of Severely Errored Frames: 0 Count of Loss of Signal Frames: 0 Remote Counters: Interleaved RS Corrected Bytes: 0 Interleaved Symbols with CRC Errors: 1 No Cell Delineation Interleaved: 0 Header Error Check Counter Interleaved: 0 Count of Severely Errored Frames: 0 Count of Loss of Signal Frames: 0 cbos#show int wan0-0 WAN0-0 ATM Logical Port PVC (VPI 0, VCI 32) is configured. ScalaRate set to Auto AAL 5 UBR Traffic IP Port Enabled cbos#show running Warning: traffic may pause while NVRAM is being accessed [[ CBOS = Section Start ]] NSOS MD5 Enable Password = XXXX NSOS MD5 Root Password = XXXX NSOS MD5 Commander Password = XXXX [[ PPP Device Driver = Section Start ]] PPP Port User Name = 00, "XXXX" PPP Port User Password = 00, XXXX PPP Port Option = 00, IPCP,IP Address,3,Auto,Negotiation Not Required,Negotiable ,IP,0.0.0.0 PPP Port Option = 00, IPCP,Primary DNS Server,129,Auto,Negotiation Not Required, Negotiable,IP,0.0.0.0 PPP Port Option = 00, IPCP,Secondary DNS Server,131,Auto,Negotiation Not Require d,Negotiable,IP,0.0.0.0 [[ ATM WAN Device Driver = Section Start ]] ATM WAN Virtual Connection Parms = 00, 0, 32, 0 [[ DHCP = Section Start ]] DHCP Server = enabled [[ IP Routing = Section Start ]] IP NAT = enabled [[ WEB = Section Start ]] WEB = enabled cbos# wtf...? Thank you all very much for taking the time to read this, and the help.

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  • File Server - Storage configuration: RAID vs LVM vs ZFS something else... ?

    - by privatehuff
    We are a small company that does video editing, among other things, and need a place to keep backup copies of large media files and make it easy to share them. I've got a box set up with Ubuntu Server and 4 x 500 GB drives. They're currently set up with Samba as four shared folders that Mac/Windows workstations can see fine, but I want a better solution. There are two major reasons for this: 500 GB is not really big enough (some projects are larger) It is cumbersome to manage the current setup, because individual hard drives have different amounts of free space and duplicated data (for backup). It is confusing now and that will only get worse once there are multiple servers. ("the project is on sever2 in share4" etc) So, I need a way to combine hard drives in such a way as to avoid complete data loss with the failure of a single drive, and so users see only a single share on each server. I've done linux software RAID5 and had a bad experience with it, but would try it again. LVM looks ok but it seems like no one uses it. ZFS seems interesting but it is relatively "new". What is the most efficient and least risky way to to combine the hdd's that is convenient for my users? Edit: The Goal here is basically to create servers that contain an arbitrary number of hard drives but limit complexity from an end-user perspective. (i.e. they see one "folder" per server) Backing up data is not an issue here, but how each solution responds to hardware failure is a serious concern. That is why I lump RAID, LVM, ZFS, and who-knows-what together. My prior experience with RAID5 was also on an Ubuntu Server box and there was a tricky and unlikely set of circumstances that led to complete data loss. I could avoid that again but was left with a feeling that I was adding an unnecessary additional point of failure to the system. I haven't used RAID10 but we are on commodity hardware and the most data drives per box is pretty much fixed at 6. We've got a lot of 500 GB drives and 1.5 TB is pretty small. (Still an option for at least one server, however) I have no experience with LVM and have read conflicting reports on how it handles drive failure. If a (non-striped) LVM setup could handle a single drive failing and only loose whichever files had a portion stored on that drive (and stored most files on a single drive only) we could even live with that. But as long as I have to learn something totally new, I may as well go all the way to ZFS. Unlike LVM, though, I would also have to change my operating system (?) so that increases the distance between where I am and where I want to be. I used a version of solaris at uni and wouldn't mind it terribly, though. On the other end on the IT spectrum, I think I may also explore FreeNAS and/or Openfiler, but that doesn't really solve the how-to-combine-drives issue.

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  • Clarification On Write-Caching Policy, Its Underlying Options And How It Applies To Hard Drives And Solid-State Drives

    - by Boris_yo
    In last week after doing more research on subject matter, I have been wondering about what I have been neglecting all those years to understand write-caching policy, always leaving it on default setting. Write-caching policy improves writing performance and consists of write-back caching and write-cache buffer flushing. This is how I understand all the above, but correct me if I erred somewhere: Write-through cache / Write-through caching itself is not a part of write caching policy per se and it's when data is written to both cache and storage device so if Windows will need that data later again, it is retrieved from cache and not from storage device which means only improved read performance as there is no need for waiting for storage device to read required data again. Since data is still written to storage device, write performance isn't improved and represents no risk of data loss or corruption in case of power failure or system crash while only data in cache gets lost. This option seems to be enabled by default and is recommended for removable devices with no need to use function of "Safely Remove Hardware" on user's part. Write-back caching is similar to above but without writing data to storage device, periodically releasing data from cache and writing to storage device when it is idle. In my opinion this option improves both read and write performance but represents risk if power failure or system crash occurs with the outcome of not only losing data eventually to be written to storage device, but causing file inconsistencies or corrupted file system. Write-back caching cannot be enabled together with write-through caching and it is not recommended to be enabled if no backup power supply is availabe. Write-cache buffer flushing I reckon is similar to write-back caching but enables immediate release and writing of data from cache to storage device right before power outage occurs but I don't know if it applies also to occasional system crash. This option seem to be complementary to write-back cache reducing or potentially eliminating risk of data loss or corruption of file system. I have questions about relevance of last 2 options to today's modern SSDs in order to get best performance and with less wear on SSDs: I know that traditional hard drives come with onboard cache (I wonder what type of cache that is), but do SSDs also come with cache? Assuming they do, is this cache faster than their NAND flash and system RAM and worth taking the risk of utilizing it by enabling write-back cache? I read somewhere that generally storage device's cache is faster than RAM, but I want to be sure. Additionally I read that write-caching should be enabled since current data that is to be written later to NAND flash is kept for a while in cache and provided there is data that gets modified a lot before finally being written, holding of this data and its periodic release reduces its write times to SSD thereby reducing its wearing. Now regarding to write-cache buffer flushing, I heard that SSD controllers are so fast by themselves that enabling this option is not required, because they manage flushing. However, once again, I don't know if SSDs have their own onboard cache and whether or not it is faster than their NAND flash and system RAM because if it is, keeping this option enabled would make sense. Recently I have posted question about issue with my Intel 330 SSD 120GB which was main reason to do deeper research having suspicion of write-caching policy being the culprit of SSD's freezing issue assuming data being released is what causes freezes. Currently I have write-cache enabled and write-cache buffer flushing disabled because I believe SSD controller's management of write-cache flushing and Windows write-cache buffer flushing are conflicting with each other: Since I want to troubleshoot in small steps to finally determine the source of issue, I have decided to start with write-caching policy and the move to drivers, switching to AHCI later on and finally disabling DIPM (device initiated power management) through registry modification thanks to @TomWijsman

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  • A failed disk (Pay for professional service or SpinRite?)(new edit)

    - by huggie
    EDIT: After much negotiating and begging and seeing through promotion smoke screen, thanks to the nice representative who took my case, I now know that the engineer has already fixed my NTFS partition (I guess it might be a bad block in the partition table?). She told me that the problem was considered minor, and I should be able to boot normally and just copy stuff out. Whew..I'm glad I didn't agree to the NTD $16,000 deal. New question (should this be in a new thread?): is it safer to use the linux "dd" command or is it better to boot normally into Windows XP and just copy stuff out? EDIT2: Thanks to all the help. I give the best answer to Console as it's most directed related to my question. But many suggestion are helpful and informational. ---- ORIGINAL POST BELOW --- Hi, in my previous post (You don't need to read but it's at http://superuser.com/questions/48838/windows-xp-a-disk-read-error-occurred), I said that my hard disk was not booting and is showing "a disk read error occurred". I took it to a recovery professional. A representative responded today told me that the NTFS partitions have a "NTFS partition system crash". I have no idea what that means. The engineer handling my drive will not be available for contact till tomorrow. Now the company charges me NTD (New Taiwan Dollar) $16,000 to recover lost data, that's kind of a lot considering that my graduate student monthly stipend is currently NTD $32,000 (max. allowed by regulation, may be lower, may change depend on funding). Now I'm weighting in between the options. Option A: let the professional recovers it with the half of my monthly stipend. If file/directories I designated are not recovered I don't pay a penny. (other than the initial examination fee of NTD $1000 which I've already paid.) Option B: let me try SpinRite, if failed, back to Option A. I spoke to the representative at the company they recommended me not to handle it on my own (yeah of course that's what they all want to say, right?), and at the price tag the disk error is probably relatively minor and data recoverable. But the representative really did not have detailed information of the disk failure so I didn't take her recommendation readily. Though one thing I heed was that she said that what they would do is to duplicate the disk before attempting discovery, so there would be no data loss (Is this true? can't duplicating invoke further data loss?). That sounds very good to me. Or maybe a third option: Option C: Negotiate with them to pay them to duplicate the disk hopefully for a much smaller price tag. Let me try SpinRite, if failed, back to Option A. This is a difficult decision. Ultimately I want my data back, but if a cheaper way is available to achieve the same thing... Can operating with SpinRite also corrupt data in someway? I've no idea what happened to my drive. I'll attempt to contact the engineer and hope to get it clarified and make an edit here.

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  • SQL SERVER – How to Recover SQL Database Data Deleted by Accident

    - by Pinal Dave
    In Repair a SQL Server database using a transaction log explorer, I showed how to use ApexSQL Log, a SQL Server transaction log viewer, to recover a SQL Server database after a disaster. In this blog, I’ll show you how to use another SQL Server disaster recovery tool from ApexSQL in a situation when data is accidentally deleted. You can download ApexSQL Recover here, install, and play along. With a good SQL Server disaster recovery strategy, data recovery is not a problem. You have a reliable full database backup with valid data, a full database backup and subsequent differential database backups, or a full database backup and a chain of transaction log backups. But not all situations are ideal. Here we’ll address some sub-optimal scenarios, where you can still successfully recover data. If you have only a full database backup This is the least optimal SQL Server disaster recovery strategy, as it doesn’t ensure minimal data loss. For example, data was deleted on Wednesday. Your last full database backup was created on Sunday, three days before the records were deleted. By using the full database backup created on Sunday, you will be able to recover SQL database records that existed in the table on Sunday. If there were any records inserted into the table on Monday or Tuesday, they will be lost forever. The same goes for records modified in this period. This method will not bring back modified records, only the old records that existed on Sunday. If you restore this full database backup, all your changes (intentional and accidental) will be lost and the database will be reverted to the state it had on Sunday. What you have to do is compare the records that were in the table on Sunday to the records on Wednesday, create a synchronization script, and execute it against the Wednesday database. If you have a full database backup followed by differential database backups Let’s say the situation is the same as in the example above, only you create a differential database backup every night. Use the full database backup created on Sunday, and the last differential database backup (created on Tuesday). In this scenario, you will lose only the data inserted and updated after the differential backup created on Tuesday. If you have a full database backup and a chain of transaction log backups This is the SQL Server disaster recovery strategy that provides minimal data loss. With a full chain of transaction logs, you can recover the SQL database to an exact point in time. To provide optimal results, you have to know exactly when the records were deleted, because restoring to a later point will not bring back the records. This method requires restoring the full database backup first. If you have any differential log backup created after the last full database backup, restore the most recent one. Then, restore transaction log backups, one by one, it the order they were created starting with the first created after the restored differential database backup. Now, the table will be in the state before the records were deleted. You have to identify the deleted records, script them and run the script against the original database. Although this method is reliable, it is time-consuming and requires a lot of space on disk. How to easily recover deleted records? The following solution enables you to recover SQL database records even if you have no full or differential database backups and no transaction log backups. To understand how ApexSQL Recover works, I’ll explain what happens when table data is deleted. Table data is stored in data pages. When you delete table records, they are not immediately deleted from the data pages, but marked to be overwritten by new records. Such records are not shown as existing anymore, but ApexSQL Recover can read them and create undo script for them. How long will deleted records stay in the MDF file? It depends on many factors, as time passes it’s less likely that the records will not be overwritten. The more transactions occur after the deletion, the more chances the records will be overwritten and permanently lost. Therefore, it’s recommended to create a copy of the database MDF and LDF files immediately (if you cannot take your database offline until the issue is solved) and run ApexSQL Recover on them. Note that a full database backup will not help here, as the records marked for overwriting are not included in the backup. First, I’ll delete some records from the Person.EmailAddress table in the AdventureWorks database.   I can delete these records in SQL Server Management Studio, or execute a script such as DELETE FROM Person.EmailAddress WHERE BusinessEntityID BETWEEN 70 AND 80 Then, I’ll start ApexSQL Recover and select From DELETE operation in the Recovery tab.   In the Select the database to recover step, first select the SQL Server instance. If it’s not shown in the drop-down list, click the Server icon right to the Server drop-down list and browse for the SQL Server instance, or type the instance name manually. Specify the authentication type and select the database in the Database drop-down list.   In the next step, you’re prompted to add additional data sources. As this can be a tricky step, especially for new users, ApexSQL Recover offers help via the Help me decide option.   The Help me decide option guides you through a series of questions about the database transaction log and advises what files to add. If you know that you have no transaction log backups or detached transaction logs, or the online transaction log file has been truncated after the data was deleted, select No additional transaction logs are available. If you know that you have transaction log backups that contain the delete transactions you want to recover, click Add transaction logs. The online transaction log is listed and selected automatically.   Click Add if to add transaction log backups. It would be best if you have a full transaction log chain, as explained above. The next step for this option is to specify the time range.   Selecting a small time range for the time of deletion will create the recovery script just for the accidentally deleted records. A wide time range might script the records deleted on purpose, and you don’t want that. If needed, you can check the script generated and manually remove such records. After that, for all data sources options, the next step is to select the tables. Be careful here, if you deleted some data from other tables on purpose, and don’t want to recover them, don’t select all tables, as ApexSQL Recover will create the INSERT script for them too.   The next step offers two options: to create a recovery script that will insert the deleted records back into the Person.EmailAddress table, or to create a new database, create the Person.EmailAddress table in it, and insert the deleted records. I’ll select the first one.   The recovery process is completed and 11 records are found and scripted, as expected.   To see the script, click View script. ApexSQL Recover has its own script editor, where you can review, modify, and execute the recovery script. The insert into statements look like: INSERT INTO Person.EmailAddress( BusinessEntityID, EmailAddressID, EmailAddress, rowguid, ModifiedDate) VALUES( 70, 70, N'[email protected]' COLLATE SQL_Latin1_General_CP1_CI_AS, 'd62c5b4e-c91f-403f-b630-7b7e0fda70ce', '20030109 00:00:00.000' ); To execute the script, click Execute in the menu.   If you want to check whether the records are really back, execute SELECT * FROM Person.EmailAddress WHERE BusinessEntityID BETWEEN 70 AND 80 As shown, ApexSQL Recover recovers SQL database data after accidental deletes even without the database backup that contains the deleted data and relevant transaction log backups. ApexSQL Recover reads the deleted data from the database data file, so this method can be used even for databases in the Simple recovery model. Besides recovering SQL database records from a DELETE statement, ApexSQL Recover can help when the records are lost due to a DROP TABLE, or TRUNCATE statement, as well as repair a corrupted MDF file that cannot be attached to as SQL Server instance. You can find more information about how to recover SQL database lost data and repair a SQL Server database on ApexSQL Solution center. There are solutions for various situations when data needs to be recovered. Reference: Pinal Dave (http://blog.sqlauthority.com)Filed under: PostADay, SQL, SQL Authority, SQL Backup and Restore, SQL Query, SQL Server, SQL Tips and Tricks, T SQL

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  • Is RTD Stateless or Stateful?

    - by [email protected]
    Yes.   A stateless service is one where each request is an independent transaction that can be processed by any of the servers in a cluster.  A stateful service is one where state is kept in a server's memory from transaction to transaction, thus necessitating the proper routing of requests to the right server. The main advantage of stateless systems is simplicity of design. The main advantage of stateful systems is performance. I'm often asked whether RTD is a stateless or stateful service, so I wanted to clarify this issue in depth so that RTD's architecture will be properly understood. The short answer is: "RTD can be configured as a stateless or stateful service." The performance difference between stateless and stateful systems can be very significant, and while in a call center implementation it may be reasonable to use a pure stateless configuration, a web implementation that produces thousands of requests per second is practically impossible with a stateless configuration. RTD's performance is orders of magnitude better than most competing systems. RTD was architected from the ground up to achieve this performance. Features like automatic and dynamic compression of prediction models, automatic translation of metadata to machine code, lack of interpreted languages, and separation of model building from decisioning contribute to achieving this performance level. Because  of this focus on performance we decided to have RTD's default configuration work in a stateful manner. By being stateful RTD requests are typically handled in a few milliseconds when repeated requests come to the same session. Now, those readers that have participated in implementations of RTD know that RTD's architecture is also focused on reducing Total Cost of Ownership (TCO) with features like automatic model building, automatic time windows, automatic maintenance of database tables, automatic evaluation of data mining models, automatic management of models partitioned by channel, geography, etcetera, and hot swapping of configurations. How do you reconcile the need for a low TCO and the need for performance? How do you get the performance of a stateful system with the simplicity of a stateless system? The answer is that you make the system behave like a stateless system to the exterior, but you let it automatically take advantage of situations where being stateful is better. For example, one of the advantages of stateless systems is that you can route a message to any server in a cluster, without worrying about sending it to the same server that was handling the session in previous messages. With an RTD stateful configuration you can still route the message to any server in the cluster, so from the point of view of the configuration of other systems, it is the same as a stateless service. The difference though comes in performance, because if the message arrives to the right server, RTD can serve it without any external access to the session's state, thus tremendously reducing processing time. In typical implementations it is not rare to have high percentages of messages routed directly to the right server, while those that are not, are easily handled by forwarding the messages to the right server. This architecture usually provides the best of both worlds with performance and simplicity of configuration.   Configuring RTD as a pure stateless service A pure stateless configuration requires session data to be persisted at the end of handling each and every message and reloading that data at the beginning of handling any new message. This is of course, the root of the inefficiency of these configurations. This is also the reason why many "stateless" implementations actually do keep state to take advantage of a request coming back to the same server. Nevertheless, if the implementation requires a pure stateless decision service, this is easy to configure in RTD. The way to do it is: Mark every Integration Point to Close the session at the end of processing the message In the Session entity persist the session data on closing the session In the session entity check if a persisted version exists and load it An excellent solution for persisting the session data is Oracle Coherence, which provides a high performance, distributed cache that minimizes the performance impact of persisting and reloading the session. Alternatively, the session can be persisted to a local database. An interesting feature of the RTD stateless configuration is that it can cope with serializing concurrent requests for the same session. For example, if a web page produces two requests to the decision service, these requests could come concurrently to the decision services and be handled by different servers. Most stateless implementation would have the two requests step onto each other when saving the state, or fail one of the messages. When properly configured, RTD will make one message wait for the other before processing.   A Word on Context Using the context of a customer interaction typically significantly increases lift. For example, offer success in a call center could double if the context of the call is taken into account. For this reason, it is important to utilize the contextual information in decision making. To make the contextual information available throughout a session it needs to be persisted. When there is a well defined owner for the information then there is no problem because in case of a session restart, the information can be easily retrieved. If there is no official owner of the information, then RTD can be configured to persist this information.   Once again, RTD provides flexibility to ensure high performance when it is adequate to allow for some loss of state in the rare cases of server failure. For example, in a heavy use web site that serves 1000 pages per second the navigation history may be stored in the in memory session. In such sites it is typical that there is no OLTP that stores all the navigation events, therefore if an RTD server were to fail, it would be possible for the navigation to that point to be lost (note that a new session would be immediately established in one of the other servers). In most cases the loss of this navigation information would be acceptable as it would happen rarely. If it is desired to save this information, RTD would persist it every time the visitor navigates to a new page. Note that this practice is preferred whether RTD is configured in a stateless or stateful manner.  

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  • DTracing TCP congestion control

    - by user12820842
    In a previous post, I showed how we can use DTrace to probe TCP receive and send window events. TCP receive and send windows are in effect both about flow-controlling how much data can be received - the receive window reflects how much data the local TCP is prepared to receive, while the send window simply reflects the size of the receive window of the peer TCP. Both then represent flow control as imposed by the receiver. However, consider that without the sender imposing flow control, and a slow link to a peer, TCP will simply fill up it's window with sent segments. Dealing with multiple TCP implementations filling their peer TCP's receive windows in this manner, busy intermediate routers may drop some of these segments, leading to timeout and retransmission, which may again lead to drops. This is termed congestion, and TCP has multiple congestion control strategies. We can see that in this example, we need to have some way of adjusting how much data we send depending on how quickly we receive acknowledgement - if we get ACKs quickly, we can safely send more segments, but if acknowledgements come slowly, we should proceed with more caution. More generally, we need to implement flow control on the send side also. Slow Start and Congestion Avoidance From RFC2581, let's examine the relevant variables: "The congestion window (cwnd) is a sender-side limit on the amount of data the sender can transmit into the network before receiving an acknowledgment (ACK). Another state variable, the slow start threshold (ssthresh), is used to determine whether the slow start or congestion avoidance algorithm is used to control data transmission" Slow start is used to probe the network's ability to handle transmission bursts both when a connection is first created and when retransmission timers fire. The latter case is important, as the fact that we have effectively lost TCP data acts as a motivator for re-probing how much data the network can handle from the sending TCP. The congestion window (cwnd) is initialized to a relatively small value, generally a low multiple of the sending maximum segment size. When slow start kicks in, we will only send that number of bytes before waiting for acknowledgement. When acknowledgements are received, the congestion window is increased in size until cwnd reaches the slow start threshold ssthresh value. For most congestion control algorithms the window increases exponentially under slow start, assuming we receive acknowledgements. We send 1 segment, receive an ACK, increase the cwnd by 1 MSS to 2*MSS, send 2 segments, receive 2 ACKs, increase the cwnd by 2*MSS to 4*MSS, send 4 segments etc. When the congestion window exceeds the slow start threshold, congestion avoidance is used instead of slow start. During congestion avoidance, the congestion window is generally updated by one MSS for each round-trip-time as opposed to each ACK, and so cwnd growth is linear instead of exponential (we may receive multiple ACKs within a single RTT). This continues until congestion is detected. If a retransmit timer fires, congestion is assumed and the ssthresh value is reset. It is reset to a fraction of the number of bytes outstanding (unacknowledged) in the network. At the same time the congestion window is reset to a single max segment size. Thus, we initiate slow start until we start receiving acknowledgements again, at which point we can eventually flip over to congestion avoidance when cwnd ssthresh. Congestion control algorithms differ most in how they handle the other indication of congestion - duplicate ACKs. A duplicate ACK is a strong indication that data has been lost, since they often come from a receiver explicitly asking for a retransmission. In some cases, a duplicate ACK may be generated at the receiver as a result of packets arriving out-of-order, so it is sensible to wait for multiple duplicate ACKs before assuming packet loss rather than out-of-order delivery. This is termed fast retransmit (i.e. retransmit without waiting for the retransmission timer to expire). Note that on Oracle Solaris 11, the congestion control method used can be customized. See here for more details. In general, 3 or more duplicate ACKs indicate packet loss and should trigger fast retransmit . It's best not to revert to slow start in this case, as the fact that the receiver knew it was missing data suggests it has received data with a higher sequence number, so we know traffic is still flowing. Falling back to slow start would be excessive therefore, so fast recovery is used instead. Observing slow start and congestion avoidance The following script counts TCP segments sent when under slow start (cwnd ssthresh). #!/usr/sbin/dtrace -s #pragma D option quiet tcp:::connect-request / start[args[1]-cs_cid] == 0/ { start[args[1]-cs_cid] = 1; } tcp:::send / start[args[1]-cs_cid] == 1 && args[3]-tcps_cwnd tcps_cwnd_ssthresh / { @c["Slow start", args[2]-ip_daddr, args[4]-tcp_dport] = count(); } tcp:::send / start[args[1]-cs_cid] == 1 && args[3]-tcps_cwnd args[3]-tcps_cwnd_ssthresh / { @c["Congestion avoidance", args[2]-ip_daddr, args[4]-tcp_dport] = count(); } As we can see the script only works on connections initiated since it is started (using the start[] associative array with the connection ID as index to set whether it's a new connection (start[cid] = 1). From there we simply differentiate send events where cwnd ssthresh (congestion avoidance). Here's the output taken when I accessed a YouTube video (where rport is 80) and from an FTP session where I put a large file onto a remote system. # dtrace -s tcp_slow_start.d ^C ALGORITHM RADDR RPORT #SEG Slow start 10.153.125.222 20 6 Slow start 138.3.237.7 80 14 Slow start 10.153.125.222 21 18 Congestion avoidance 10.153.125.222 20 1164 We see that in the case of the YouTube video, slow start was exclusively used. Most of the segments we sent in that case were likely ACKs. Compare this case - where 14 segments were sent using slow start - to the FTP case, where only 6 segments were sent before we switched to congestion avoidance for 1164 segments. In the case of the FTP session, the FTP data on port 20 was predominantly sent with congestion avoidance in operation, while the FTP session relied exclusively on slow start. For the default congestion control algorithm - "newreno" - on Solaris 11, slow start will increase the cwnd by 1 MSS for every acknowledgement received, and by 1 MSS for each RTT in congestion avoidance mode. Different pluggable congestion control algorithms operate slightly differently. For example "highspeed" will update the slow start cwnd by the number of bytes ACKed rather than the MSS. And to finish, here's a neat oneliner to visually display the distribution of congestion window values for all TCP connections to a given remote port using a quantization. In this example, only port 80 is in use and we see the majority of cwnd values for that port are in the 4096-8191 range. # dtrace -n 'tcp:::send { @q[args[4]-tcp_dport] = quantize(args[3]-tcps_cwnd); }' dtrace: description 'tcp:::send ' matched 10 probes ^C 80 value ------------- Distribution ------------- count -1 | 0 0 |@@@@@@ 5 1 | 0 2 | 0 4 | 0 8 | 0 16 | 0 32 | 0 64 | 0 128 | 0 256 | 0 512 | 0 1024 | 0 2048 |@@@@@@@@@ 8 4096 |@@@@@@@@@@@@@@@@@@@@@@@@@@ 23 8192 | 0

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  • To SYNC or not to SYNC – Part 3

    - by AshishRay
    I can't believe it has been almost a year since my last blog post. I know, that's an absolute no-no in the blogosphere. And I know that "I have been busy" is not a good excuse. So - without trying to come up with an excuse - let me state this - my apologies for taking such a long time to write the next Part. Without further ado, here goes. This is Part 3 of a multi-part blog article where we are discussing various aspects of setting up Data Guard synchronous redo transport (SYNC). In Part 1 of this article, I debunked the myth that Data Guard SYNC is similar to a two-phase commit operation. In Part 2, I discussed the various ways that network latency may or may not impact a Data Guard SYNC configuration. In this article, I will talk in details regarding why Data Guard SYNC is a good thing. I will also talk about distance implications for setting up such a configuration. So, Why Good? Why is Data Guard SYNC a good thing? Because, at the end of the day, this gives you the assurance of zero data loss - it doesn’t matter what outage may befall your primary system. Befall! Boy, that sounds theatrical. But seriously - think about this - it minimizes your data risks. That’s a big deal. Whether you have an outage due to bad disks, faulty hardware components, hardware / software bugs, physical data corruptions, power failures, lightning that takes out significant part of your data center, fire that melts your assets, water leakage from the cooling system, human errors such as accidental deletion of online redo log files - it doesn’t matter - you can have that “Om - peace” look on your face and then you can failover to the standby system, without losing a single bit of data in your Oracle database. You will be a hero, as shown in this not so imaginary conversation: IT Manager: Well, what’s the status? You: John is doing the trace analysis on the storage array. IT Manager: So? How long is that gonna take? You: Well, he is stuck, waiting for a response from <insert your not-so-favorite storage vendor here>. IT Manager: So, no root cause yet? You: I told you, he is stuck. We have escalated with their Support, but you know how long these things take. IT Manager: Darn it - the site is down! You: Not really … IT Manager: What do you mean? You: John is stuck, but Sreeni has already done a failover to the Data Guard standby. IT Manager: Whoa, whoa - wait! Failover means we lost some data, why did you do this without letting the Business group know? You: We didn’t lose any data. Remember, we had set up Data Guard with SYNC? So now, any problems on the production – we just failover. No data loss, and we are up and running in minutes. The Business guys don’t need to know. IT Manager: Wow! Are we great or what!! You: I guess … Ok, so you get it - SYNC is good. But as my dear friend Larry Carpenter says, “TANSTAAFL”, or "There ain't no such thing as a free lunch". Yes, of course - investing in Data Guard SYNC means that you have to invest in a low-latency network, you have to monitor your applications and database especially in peak load conditions, and you cannot under-provision your standby systems. But all these are good and necessary things, if you are supporting mission-critical apps that are supposed to be running 24x7. The peace of mind that this investment will give you is priceless, especially if you are serious about HA. How Far Can We Go? Someone may say at this point - well, I can’t use Data Guard SYNC over my coast-to-coast deployment. Most likely - true. So how far can you go? Well, we have customers who have deployed Data Guard SYNC over 300+ miles! Does this mean that you can also deploy over similar distances? Duh - no! I am going to say something here that most IT managers don’t like to hear - “It depends!” It depends on your application design, application response time / throughput requirements, network topology, etc. However, because of the optimal way we do SYNC, customers have been able to stretch Data Guard SYNC deployments over longer distances compared to traditional, storage-centric ways of doing this. The MAA Database 10.2 best practices paper Data Guard Redo Transport & Network Configuration, and Oracle Database 11.2 High Availability Best Practices Manual talk about some of these SYNC-related metrics. For example, a test deployment of Data Guard SYNC over 330 miles with 10ms latency showed an impact less than 5% for a busy OLTP application. Even if you can’t deploy Data Guard SYNC over your WAN distance, or if you already have an ASYNC standby located 1000-s of miles away, here’s another nifty way to boost your HA. Have a local standby, configured SYNC. How local is “local”? Again - it depends. One customer runs a local SYNC standby across the campus. Another customer runs it across 15 miles in another data center. Both of these customers are running Data Guard SYNC as their HA standard. If a localized outage affects their primary system, no problem! They have all the data available on the standby, to which they can failover. Very fast. In seconds. Wait - did I say “seconds”? Yes, Virginia, there is a Santa Claus. But you have to wait till the next blog article to find out more. I assure you tho’ that this time you won’t have to wait for another year for this.

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  • RAID 0 Volatile Volume Cache Mode configuration

    - by SnippetSpace
    I discovered that in IRST there is an option to set a cache mode for my 3 ssd raid 0 array. I've read the documentation by Intel and have some questions: Are there any overall benefits/risks from enabling cache mode? As I'm on a laptop, would write back be recommended? I read it increases chance of data loss on power interruption. What is the difference between how windows handles data integrity and the intel driver? Read only mode seems to have the benefit of faster reads, does it have any downsides? Thanks for your help guys!

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  • Windows 2003 VM, not connecting to VM Network

    - by TheWellington
    I am running VMware's vSphere infrastructure. I have a windows 2003 VM that is suddenly not connected to the network. I can log into vSphere and see that the VM is running, but it does not connect to the VM network. The firewall on this VM is not running. The network adapter in the VM is configured correctly. The only evidence I see indicating an issue is in the event viewer. I have the following entry. Source: VMUpgradeHelper EventID: 270 Description: Not restoring network configuration for adapter with MAC address 00:50:56:xx:xx:xx. The device ID for this adapter is unchanged. THis VM is a webserver, and it was working beautifully just two days ago. "nothing" has happened... so I am at a loss as to what may have happened. Ideas??

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  • PXE E52 proxyDHCP offers were received. No DHCP offers were received

    - by TonyP
    Have a Lenovo laptop which i'm trying to capture an image from as a test (we are moving from HP as our std laptop) and i keep getting the PXE E52 error when trying to boot from LAN. With things exactly as they are all the previous HP models are connecting ok. I'm at a loss as to what to test, as its working for the HP kit and don't want to break that. Note: DHCP server and WDS server are two separate boxes but on same scope, VLAN, etc. and the DHCP lease is not full.

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  • osx split external hard drive partition

    - by Bart
    Hi, I currently have a 640GB external HD that has 1 partition formatted as HSF+ Now I want to split some of the free space into a new FAT32 partition, without having to reformat the whole HD and losing all my data. I read that I'm supposed to be able to add new partitions in the Hard Disc Utility by clicking the "+" sign, without any loss of data. But in my case the "+" is not clickable and it says that this partition cannot be altered. Can anyone tell me how to proceed. Or is it impossible without reformatting the whole disc? Thanks ps: I'm running osx snow leopard 10.6.6

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  • Trouble using Upstart to launch Redis as redis user

    - by Chris
    I'm trying to launch redis-server as a user (called redis) via Upstart. My /etc/init/redis-server.conf looks like this: description "redis server" start on runlevel [23] stop on shutdown exec sudo -u redis /usr/local/bin/redis-server /var/lib/redis/redis.conf Looks good, right? I start redis-server using $start redis-server redis-server start/running, process 16808 $redis-cli Could not connect to Redis at 127.0.0.1:6379: Connection refused $ps ax | grep ps 168 16810 tty1 R+ 0:00 ps ax 16811 tty1 S+ 0:00 grep 168 So redis-server definitely isn't running. Let's try executing the Upstart command by hand, shall we? exec sudo -u redis /usr/local/bin/redis-server /var/lib/redis/redis.conf [16852] 19 Jun 10:37:21 # Can't chdir to './': Permission denied Connection to 10.19.2.94 closed. And then I get logged off. I'm at a loss. Any ideas?

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  • OpenLDAP and Samba, can't log onto Samba share from Windows

    - by Jakobud
    The former jackass IT-guy that I'm taking over for had a Samba share setup on a Fedora server that uses our OpenLDAP server to authenticate users who want to log in from Windows. We recently added a new employee and I jumped through the LDAP hoops to add them to the system. However, I can't seem to use their login to access the Samba share. I'm looking through the LDAP settings and Groups and comparing the new user account to existing ones, and I can't figure out what settings in LDAP are required for this user to be able to access the Samba share. Of course the former idiotic IT-guy didn't document a single thing and has all sorts of weird setups on the network. So I'm at a bit of a loss on knowing what to look for here. Where should I start? On the server that is hosting the Samba share, he has samba running obviously but also has smbldap-tools loaded as well.

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  • Using mod_rewrite for a RESTful api

    - by razass
    Say the user is making a request to the following url: "http://api.example.com/houses/123/abc" That request needs to map to "/webroot/index.php" and 'houses', '123', 'abc' need to be able to be parsed out of the URL in that index.php. It also can't alter the http headers or body. There can be any number of variables after the domain ie) "http://api.example.com/houses/1234/abc/zxy/987" I think I already have all requests being sent to webroot using: <IfModule mod_rewrite.c> RewriteEngine on RewriteCond $0 !^webroot/ RewriteRule .* webroot/$0 [L] </IfModule> Which appears to be working but I am not sure if it is correct. But now I am at a loss as to how to take the next step as mentioned above. Thanks in advance!

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  • Windows 2003 VM, not connecting to VM Network

    - by TheWellington
    I am running VMware's vSphere infrastructure. I have a windows 2003 VM that is suddenly not connected to the network. I can log into vSphere and see that the VM is running, but it does not connect to the VM network. The firewall on this VM is not running. The network adapter in the VM is configured correctly. The only evidence I see indicating an issue is in the event viewer. I have the following entry. Source: VMUpgradeHelper EventID: 270 Description: Not restoring network configuration for adapter with MAC address 00:50:56:xx:xx:xx. The device ID for this adapter is unchanged. THis VM is a webserver, and it was working beautifully just two days ago. "nothing" has happened... so I am at a loss as to what may have happened. Ideas??

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