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• Mounting Replicated Gluster Multi-AZ Storage

  - by Roman Newaza

  I have Replicated Gluster Storage which is used by Auto scaling Servers. Both, Auto scaling and Storage are allocated in two Availability zones. Gluster: Number of Bricks: 4 x 2 = 8 Transport-type: tcp Bricks: Brick1: gluster01:/storage/1a # Zone A Brick2: gluster02:/storage/1b # Zone B Brick3: gluster03:/storage/2a # Zone A Brick4: gluster04:/storage/2b # Zone B Brick5: gluster01:/storage/3a # Zone A Brick6: gluster02:/storage/3b # Zone B Brick7: gluster03:/storage/4a # Zone A Brick8: gluster04:/storage/4b # Zone B I used Round Robin DNS for Gluster entry point, so DNS name resolves to all of the storage server addresses which are returned in different order all the time: # host storage.domain.com storage.domain.com has address xx.xx.xx.x1 storage.domain.com has address xx.xx.xx.x2 storage.domain.com has address xx.xx.xx.x3 storage.domain.com has address xx.xx.xx.x4 The Storage is mounted with Native Gluster Client: # grep storage /etc/fstab storage.domain.com:/storage /storage glusterfs defaults,log-level=WARNING,log-file=/var/log/gluster.log 0 0 I have heard Gluster might be mounted with the first Server IP and after that it will fetch its configuration with the rest of Servers. Personally, I never tested single Server mount setup and I don't know how Gluster handles this. On EC2, traffic among single Availability zone is free and between different zones is not. When Client in zone A writes to storage and IP of Storage in zone B is returned, it will cost me twice more for data transfer: Client (Zone A) - Storage Server (Zone B) - Replication to Storage Server (Zone A). Question: Would it be better to mount Storage Server of the same zone, so that data transfer charges apply only for replication (A - A - B)?

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• BGP Multihomed/Multi-location best practice

  - by Tom O'Connor

  We're in the process of designing a new iteration of our network where we improve resilliency by adding a second datacentre. We'll be adding a second datacentre, with an identical configuration of servers as our primary location. To achieve network connectivity, we're looking into a couple of possible methods. See earlier questions http://serverfault.com/questions/86736/best-way-to-improve-resilience and http://serverfault.com/questions/101582/dns-round-robin-failover-and-load-balancing I'm pretty convinced that BGP is the right way to go about this, and this question is not about RRDNS. 1) If we have 2 locations, do we announce the same IP address block from both locations? 2) If we did this, but had a management ssh interface on x.x.x.50 from datacentre A, but it was on x.x.x.150 in datacentre B. What is the best practice mechanism for achieving this? Because if I were nearest to A, then all my traffic would go to x.50, but if i attempted to connect to x.150, I'd not be able to connect, because this address wouldn't be valid at A, but only at B. Is the best solution to announce 2 different netblocks, one at each location, facilitating the need for RRDNS, or to announce a single block, and run some form of VPN between the two sites for managment traffic?

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• how to design pound -> varnish -> jboss for ha + loadbalancing

  - by andreash

  Hello, I'm planning a new infrastructure for our web application. We have two JBossAS5 servers, running in a cluster. Session state will be replicated via JBoss Cache. In front of that, there should be some cache, to speed up delivery of static elements. However, most of the traffic to our app will be via HTTPS. So far, I had been thinking of two Varnish caches in front of the JBossASs, each being configured for loadbalancing to the two JBossASs via round-robin. Since Varnish doesn't handle HTTPS, then there would need to be two pound proxies in front of the Varnishs, dealing with the HTTPS. The two pounds would be made high-available with Heartbeat/LinuxHA. The traffic to www.example.com would then be going through our firewall, from there to the virtual IP of the pounds, from there to the Varnishs, and from there to the JBossASs. Question 1: Does this make sense? Or is it overly complicated, and the same goal can be reached with simpler methods? Question 2: If my layout is fine, how do I configure the pound - Varnish step? Should I a) make the Varnish service high-available through Heartbeat/LinuxHA as well and direct traffic from pound to the virtual IP of the Varnishs, or should I rather b) Configure two independent Varnishs and use load-balancing in pound to address the different Varnishs? Thanks a lot for your insight! Andreas.

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• Multi-IP address zimbra server DNS PTR records and spam

  - by David Fraser

  We have a mail server running Zimbra (ZCS 6.0.8). The server has 5 active public IP addresses in the same subnet. (.226-.230). I currently have A records for each of these (host0.domain.com..host4.domain.com), with the main host.domain.com of the machine pointing to .226. Our host has ended up being listed on the SORBS DUHL list (even though it's in a server farm). According to them you can get removed quickly by checking that your host has an MX record, an A record, and a PTR record that points back to the hostname given in the MX record. I tried setting the PTR records so that each of these addresses resolved back to their A record (i.e. .228 had a PTR to host2.domain.com). However, I then got mail being rejected from other servers because when Postfix (under Zimbra control) sends out mail, it uses the main hostname for the HELO - there doesn't seem to be any way to override it. So the PTR records currently say host.domain.com for all 5 IP addresses. What's the correct way to handle this? Should I have an A record for the domain that points to all the IP addresses (for round-robin handling)? I'm nervous of changes that could cause problems, so I'm wondering what the standard way to handle a multiple-IP-address mail server is.

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• Managing multiple Apache proxies simultaneously (mod_proxy_balancer)

  - by Hank

  The frontend of my web application is formed by currently two Apache reverse proxies, using mod_proxy_balancer to distribute traffic over a number of backend application servers. Both frontend reverse proxies, running on separate hosts, are accessible from the internet. DNS round robin distributes traffic over both. In the future, the number of reverse proxies is likely to grow, since the webapplication is very bandwidth-heavy. My question is: how do I keep the state of both reverse balancers / proxies in sync? For example, for maintenance purposes, I might want to reduce the load on one of the backend appservers. Currently I can do that by accessing the Balancer-Manager web form on each proxy, and change the distribution rules. But I have to do that on each proxy manually and make sure I enter the same stuff. Is it possible to "link" multiple instances of mod_proxy_balancer? Or is there a tool out there that connects to a number of instances, and updates all with the same information? Update: The tool should retrieve the runtime status and make runtime changes, just like the existing Balancer-Manager, only for a number of proxies - not just for one. Modification of configuration files is not what I'm interested in (as there are plenty tools for that).

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• How many reverse proxies (nginx, haproxy) is too many?

  - by Alysum

  I'm setting up a HA (high availability) cluster using nginx, haproxy & apache. I've been reading great things about nginx and haproxy. People tend to choose one or the other but I like both. Haproxy is more flexible for load balancing than nginx's simple round robin (even with the upstream-fair patch). But I'd like to keep nginx for redirecting non-https to https among other things right at the point of entry to the cluster. On the other hand, nginx is a lot faster for serving static contents and would reduce the load on the powerful apache which loves to eat a lot of RAM! Here is my planned setup: Load balancer: nginx listens on port 80/443 and proxy_forwards to haproxy on 8080 on the same server to load balance between the multiple nodes. Nodes: nginx on the node listens to requests coming from haproxy on 8080, if the content is static, serve it. But if it's a backend script (in my case PHP), proxy forward to apache2 on the same node server listenning on a different port number. Technically this setup works but my concerns are whether having the requests going through several proxies is going to slow down requests? Most of the requests will be PHP requests as the backends are services (which means groing from nginx - haproxy - nginx - apache). Thoughts? Cheers

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• Can a pool of memcache daemons be used to share sessions more efficiently?

  - by Tom

  We are moving from a 1 webserver setup to a two webserver setup and I need to start sharing PHP sessions between the two load balanced machines. We already have memcached installed (and started) and so I was pleasantly surprized that I could accomplish sharing sessions between the new servers by changing only 3 lines in the php.ini file (the session.save_handler and session.save_path): I replaced: session.save_handler = files with: session.save_handler = memcache Then on the master webserver I set the session.save_path to point to localhost: session.save_path="tcp://localhost:11211" and on the slave webserver I set the session.save_path to point to the master: session.save_path="tcp://192.168.0.1:11211" Job done, I tested it and it works. But... Obviously using memcache means the sessions are in RAM and will be lost if a machine is rebooted or the memcache daemon crashes - I'm a little concerned by this but I am a bit more worried about the network traffic between the two webservers (especially as we scale up) because whenever someone is load balanced to the slave webserver their sessions will be fetched across the network from the master webserver. I was wondering if I could define two save_paths so the machines look in their own session storage before using the network. For example: Master: session.save_path="tcp://localhost:11211, tcp://192.168.0.2:11211" Slave: session.save_path="tcp://localhost:11211, tcp://192.168.0.1:11211" Would this successfully share sessions across the servers AND help performance? i.e save network traffic 50% of the time. Or is this technique only for failovers (e.g. when one memcache daemon is unreachable)? Note: I'm not really asking specifically about memcache replication - more about whether the PHP memcache client can peak inside each memcache daemon in a pool, return a session if it finds one and only create a new session if it doesn't find one in all the stores. As I'm writing this I'm thinking I'm asking a bit much from PHP, lol... Assume: no sticky-sessions, round-robin load balancing, LAMP servers.

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• What's the best way to run Drupal and Django sites behind the same Varnish server?

  - by Alexis Bellido

  I have a high traffic website running with Drupal and Apache, five web servers behind a Varnish server load balancing. Let's say this site is example.com. I'm using five backends and a director like this in my default.vcl: director balancer round-robin { { .backend = web1; } { .backend = web2; } { .backend = web3; } { .backend = web4; } { .backend = web5; } } Now I'm working on a new Django project that will be a new section of this site running on example.com/new-section. After checking the documentation I found I can do something like this: sub vcl_recv { if (req.url ~ "^/new-section/") { set req.backend = newbackend; } else { set req.backend = default; } } That is, using a different backend for a subdirectory /new-section under the same domain. My question is, how do I make something like this work with my director and load balancing setup? I'm probably going to run two or more web servers (backends) with my new Django project, each one with a mix of Gunicorn, Nginx, and a few Python packages, and would like to put all of those in their own Varnish director to load balance. Is it possible to do use the above approach to decide which director to use?, like this: sub vcl_recv { if (req.url ~ "^/new-section/") { set req.director = newdirector; } else { set req.director = balancer; } } All suggestions welcome. Thanks!

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• What's the piece of hardware listening on Facebook's or Wikipedia's IP address?

  - by Igor Ostrovsky

  I am trying to understand how massive sites like Facebook or Wikipedia work, for my intellectual curiosity. I read about various techniques for building scalable sites, but I am still puzzled about one particular detail. The part that confuses me is that ultimately, the DNS will map the entire domain to a single IP address, or a handful of IP addresses in the case of round-robin DNS. For example, wikipedia.org has only one type-A DNS record. So, people from all over the world visiting Wikipedia have to send a request to the one IP address specified in DNS. What is the piece of hardware that listens on the IP address for a massive site, and how can it possibly handle all the load coming from the requests for users all over the world? Edit 1: Thanks for all the responses! Anycast seems like a feasible answer... Does anyone know of a way to check whether a particular IP address is anycast-routed, so that I could verify that this really is the trick used in practice by large sites? Edit 2: After more reading on the topic, it appears that anycast is not typically used for dynamic web content. Anycast is usually used for UDP (e.g., DNS lookups), or sometimes for static content. One interesting thing to note is that Facebook uses profile.ak.fbcdn.net to host static content like style sheets and javascript libraries. Each time I ping this name, I get a response from a different IP address. However, I can't tell whether this is anycast in action, or a completely different technique. Back to my original question: as far as I can tell, even a large site will have a single expensive piece of load-balancing hardware listening on its handful of public IP addresses.

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• Insufficient channel capacity of 1GBit

  - by Roman S

  There is a Caching Server (Varnish): it receives data from Amazon S3 on request, saves it for some time and gives it to the client. We have encountered the problem of insufficient channel capacity of 1GBit. Peak load within 4 hours completely chokes the channel. Server performance is sufficient for now. Approximately 4.5TB of data are transmitted per day. More than 100TB are accumulated per month. The first thought that comes to mind is simply to add one more 1GBit port and sleep peacefully until 2GBit are not enough (it may happen quite quickly) or one server is not able to handle it. And then we just need to add new Caching Servers. But now we need a Load Balancer, which will send requests on one and the same URL, always on one and the same server (to avoid multiple copies of the same cached objects). Here are the questions: Does a Balancer need a band equal to sum of all bands of Caching Servers? What shall we do in case there are no ports in a Balancer? Should we add more Balancers or solve the problem by means of Round robin DNS? What are the standard approaches to such problems? Can anyone advise hosting-companies, which can solve this problem? We are interested in American and European markets.

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• Apache2: Limit simultaneous requests & throttle bandwidth per IP/client?

  - by xentek

  I want to limit simultaneous requests & throttle bandwidth per IP/Client on a single apache vhost. In other words, I want to ensure that this site, which hosts large media files, doesn't get hammered by someone trying to download everything all at once (just happened the other night). I'd like to limit the outgoing transfer speed overall for this site, as well as limit the number of connections a single IP can make to the server to a sane default (i.e. within normal browser limits for multiple requests so page loads aren't effected too much). Bonus points if I can actually scope it to file types (i.e. leave web files alone, but apply these rules to just the media files). We're running Ubuntu 9.04 on all the servers, and have two apache/php servers being load balanced via Round Robin by a squid proxy server. MySQL is running on its own box as well. We've got plenty of bandwidth to give them, so I don't really want overall caps, but just want to throttle the amount of memory/CPU it takes to serve this site. There other sites on these servers that we don't want to apply these rules too, just want to keep this one from hogging all the resources. Let me know if you need more info! Thanks in advance for your suggestions!

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• How browsers handle multiple IPs

  - by Sandman4

  Can someone direct me to information on exact browsers behavior when browser gets multiple A records for a given hostname (say ip1 and ip2), and one of them is not accessible. I interested in EXACT details, like (but not limited to): Will browser get 2 IPs from OS, or it will get only one ? Which ip will browser try first (random or always the first one) ? Now, let's say browser started with the failed ip1 For how long will browser try ip1 ? If user hits "stop" while it waits for ip1, and then clicks refresh which IP will browser try ? What will happen when it times-out - will it start trying ip2 or give error ? (And if error, which ip will browser try when user clicks refresh). When user clicks refresh, will any browser attempt new DNS lookup ? Now let's assume browser tried working ip2 first. For the next page request, will browser still use ip2, or it may randomly switch ips ? For how long browsers keep IPs in their cache ? When browsers sends a new DNS request, and get SAME ips, will it CONTINUE to use the same known-to-be-working IP, or the process starts from scratch and it may try any of the two ? Of course it all may be browser dependent, and may also vary between versions and platforms, I'd be happy to have maximum of details. The purpose of this - I'm trying to understand what exactly users will experience when round-robin DNS based used and one of the hosts fails. Please, I'm NOT asking about how bad DNS load balancing is, and please refrain from answering "don't do it", "it's a bad idea", "you need heartbeat/proxy/BGP/whatever" and so on.

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• Squid reverse proxy array - siblings not communicating with each other

  - by V. Romanov

  I want to set up 2 squid servers to act as reverse proxy and cache for a webserver on our intranet. The load balancing will be done with DNS round robin or just different mappings for different clients. The thing is, I want both servers to try and contact each other to see if they have the object required in cache before contacting the webserver for it (the network that servers the webserver is the bottleneck and I'm trying to eliminate it) Both squids are configured the same, here are the relevant config lines : acl dvr1_cache_it_best_tv_com dstdomain dvr1.cache.it.best-tv.com acl squid1_it_best_tv_com dstdomain squid1.it.best-tv.com acl squid2_it_best_tv_com dstdomain squid2.it.best-tv.com http_access allow dvr1_cache_it_best_tv_com http_access allow squid1_it_best_tv_com http_access allow squid2_it_best_tv_com http_access allow all http_port 8081 accel defaultsite=dvr1.cache.it.best-tv.com cache_peer dvr1.origin.it.best-tv.com parent 80 0 no-query originserver name=Proxy_dvr1_origin_it_best_tv_com cache_peer squid1.it.best-tv.com sibling 8081 3130 weight=10 name=Proxy_Squid1_it_best_tv_com cache_peer squid2.it.best-tv.com sibling 8081 3130 weight=10 name=Proxy_Squid2_it_best_tv_com cache_peer_access Proxy_dvr1_origin_it_best_tv_com allow dvr1_cache_it_best_tv_com cache_peer_access Proxy_squid1_it_best_tv_com allow squid1_it_best_tv_com cache_peer_access Proxy_squid1_it_best_tv_com allow squid2_it_best_tv_com cache_peer_access Proxy_squid1_it_best_tv_com allow dvr1_cache_it_best_tv_com cache_peer_access Proxy_squid2_it_best_tv_com allow squid1_it_best_tv_com cache_peer_access Proxy_squid2_it_best_tv_com allow squid2_it_best_tv_com cache_peer_access Proxy_squid2_it_best_tv_com allow dvr1_cache_it_best_tv_com just to make it clear - dvr1.cache is the alias for the proxy servers. dvr1.origin is the web server. Both servers work, both serve content and cache it and work fine. However, when I clear the cache on one server and then access it, it gets the content from the parent (DVR1_ORIGIN) instead of going to the sibling squid. What did I configure wrong? Or perhaps I don't understand the architecture correctly? I read the squid manuals but as far as I see i did it all by the book and yet it doesn't work right. Any help will be appreciated!

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• Good Enough Failover Strategy for DNS / MySQL / Email

  - by IMB

  I've asked and read a lot questions regarding DNS failover but the more I read the more complicated it becomes, some people say it's good enough some say it isn't. No clear answers from what I read. I was wondering if we can set it straight once and for all, at least for the requirements of most websites out there. Right now let's assume the following: We don't need really need load-balancing, what we need is a failover solution. We are running a website based on LAMP on a VPS. We need to make sure that the Web Server, MySQL, Email are always accessible if not 99%. Basically here's my idea and questions about it: Web Server: We need at least one failover server (another VPS on a separate data center). Is DNS Failover via Round Robin good, if not, what's the best? And how do you exactly implement it? How do you make the files you upload/delete on Server A is also on Server B? MySQL: I've only read a brief intro to MySQL replication and I assume that I can replicate Server A to Server B and vice versa on the fly right? So just it case Server A fails and Server B is now running, it will continue to work and replicate to Server A when it becomes available. So in essence Server B is now the primary server, and will later on failover to Server A, should a failure happen again. Email: If we are gonna use DNS Failover, using webmail or relying on emails stored on the server is probably not a good idea right? Since some emails might be on Server A while some might be on Server B? I assume a basic email forwarder to a 3rdparty is good enough (like Gmail for example) to ensure all emails are kept in one place. Here's a basic diagram for a better picture: http://i.stack.imgur.com/KWSIi.png

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• Webcam security camera software that runs as a service

  - by hurfdurf

  I've been looking for Windows webcam software that will run as a Windows service without any user login. The goal is to use the webcam as a cheap security camera and log the results to secure networked storage (windows share, not FTP). The requirements are: Motion detection Video capture Runs as a service (should start recording immediately after reboot) Nice to have: Round-robin storage, e.g. 10Gb limit, oldest files overwritten/deleted when space gets low I've read the other webcam questions but still haven't stumbled across anything suitable. Evaluations thus far: Title MotionDetect Service Snapshots Video SpaceLimit License Yawcam Yes Yes Yes No No GPL WebCam ZoneTrigger Yes No Yes Yes No Commercial Dorgem Yes No Yes Yes No GPL AbelCam Yes No Yes Yes No Commercial Logitech Yes No Yes Yes No Paired with camera IspyConnect Yes No Yes Yes Yes Free SecureCam (SourcefoYes No Yes Yes No GPL AbelCam Yes No Yes Yes No Commercial Active WebCam Yes Yes(?) Yes Yes Volume Free Commercial WebCam Surveyor Yes No Yes Yes No Commercial WebCamsPy NA NA NA NA NA GPL Camera: Logitech Webcam Pro 9000 Windows 7 32-bit WebCamsPy failed to initialize so couldn't be tested So far, the contenders: Active Webcam comes the closest, and claims to run as a service, but i haven't been able to get it to record after a cold boot even though a service is running. Yawcam can be set up as a service but doesn't record video. IspyConnect has exactly the type of space limit I want and looks great, but doesn't run as a service (seems also to be a bit of a cpu hog) Any other suggestions? I'm locked into Windows so can't use linux Motion, which looks almost perfect. Any pointers to rich Windows webcam/motion detection libraries out there that could easily be turned into a command line program would also be appreciated.

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• Nginx proxy upstream cached?

  - by Julian H. Lam

  Attempting to resolve an issue that's been annoying me for a bit. I've distilled the symptoms into a set of reproducible steps: I have two sites, siteA, and siteB. They are both Node.js applications running on different ports (for the sake of example, 4567 and 4568) Both applications have their own file in sites_available (plus a symlink from sites_enabled), which contain the directives proxy_pass http://node_siteA/ and proxy_pass http://node_siteB/ respectively, inside of a location block. They also each have an upstream block (defined globally?): upstream node_siteA { upstream node_siteB { server 127.0.0.1:4567; server 127.0.0.1:4568; } } Site A and Site B have nothing to do with each other. Yes, I am restarting (reloading, actually) nginx every time I make a change. If I take down site B and attempt to access it via the web, I am served site A. Why is this? Thoughts Other times, when I create a new Site C, for example, nginx refuses to show me anything except "Welcome to nginx!" for ~5 minutes. This suggests a resolver timeout, perhaps? When I access Site B after its config has been deleted, and it sends me to Site A, this sounds like nginx sending me to servers in a round-robin fashion...

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• Apache Server with memcache, varnish and php slow request times

  - by coolestdude1

  My issue is that these servers are taking rather long for request about 2 seconds on average just to serve files. When we had just one server doing everything it was noticeably faster even with the same web app (Drupal 6 and Drupal 7). I want to get this number down to a reasonable level and so I need some help getting to the bottom of why the request times are so slow. This can cause the webapp to hang on post or put and generally leads to a bad user experience on my sites. PS: I am more of a server newbie so this has confounded me for quite some time. The domains: collabornation.net nptrainingworks.com (they run off the same two webservers using vhost configs) The Gear: Two Rackspace 4 Gig servers running CentOS 6.2 Final They have a mounted file system (gluster) that is used to keep files the same on both machines. They are behind a rackspace load balancer running round robin. Mysql is run using php-pdo and php-mysql as such mysql is run on another instance running memcache on that machine with phpMyAdmin located there as well. Apache version number 2.2.15-15.el6.centos.1 (httpd.x86_64) Varnish version number 3.0.2-1.el5 (varnish.x86_64) PHP version number 5.3.14-1.el6.remi (php.x86_64) Configs Linked Below Apache Conf Vhost Conf Varnish Backends Varnish Defaults Varnish Acl PHP INI Again need some help, much appreciated!

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• Apache load balancer with https real servers and client certificates

  - by Jack Scheible

  Our network requirements state that ALL network traffic must be encrypted. The network configuration looks like this: ------------ /-- https --> | server 1 | / ------------ |------------| |---------------|/ ------------ | Client | --- https --> | Load Balancer | ---- https --> | server 2 | |------------| |---------------|\ ------------ \ ------------ \-- https --> | server 3 | ------------ And it has to pass client certificates. I've got a config that can do load balancing with in-the-clear real servers: <VirtualHost *:8666> DocumentRoot "/usr/local/apache/ssl_html" ServerName vmbigip1 ServerAdmin [email protected] DirectoryIndex index.html <Proxy *> Order deny,allow Allow from all </Proxy> SSLEngine on SSLProxyEngine On SSLCertificateFile /usr/local/apache/conf/server.crt SSLCertificateKeyFile /usr/local/apache/conf/server.key <Proxy balancer://mycluster> BalancerMember http://1.2.3.1:80 BalancerMember http://1.2.3.2:80 # technically we aren't blocking anyone, but could here Order Deny,Allow Deny from none Allow from all # Load Balancer Settings # A simple Round Robin load balancer. ProxySet lbmethod=byrequests </Proxy> # balancer-manager # This tool is built into the mod_proxy_balancer module allows you # to do simple mods to the balanced group via a gui web interface. <Location /balancer-manager> SetHandler balancer-manager Order deny,allow Allow from all </Location> ProxyRequests Off ProxyPreserveHost On # Point of Balance # Allows you to explicitly name the location in the site to be # balanced, here we will balance "/" or everything in the site. ProxyPass /balancer-manager ! ProxyPass / balancer://mycluster/ stickysession=JSESSIONID </VirtualHost> What I need is for the servers in my load balancer to be BalancerMember https://1.2.3.1:443 BalancerMember https://1.2.3.2:443 But that does not work. I get SSL negotiation errors. Even when I do get that to work, I will need to pass client certificates. Any help would be appreciated.

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• What are the typical methods used to scale up/out email storage servers?

  - by nareshov

  Hi, What I've tried: I have two email storage architectures. Old and new. Old: courier-imapds on several (18+) 1TB-storage servers. If one of them show signs of running out of disk space, we migrate a few email accounts to another server. the servers don't have replicas. no backups either. New: dovecot2 on a single huge server with 16TB (SATA) storage and a few SSDs we store fresh mails on the SSDs and run a doveadm purge to move mails older than a day to the SATA disks there is an identical server which has a max-15min-old rsync backup from the primary server higher-ups/management wanted to pack in as much storage as possible per server in order to minimise the cost of SSDs per server the rsync'ing is done because GlusterFS wasn't replicating well under that high small/random-IO. scaling out was expected to be done with provisioning another pair of such huge servers on facing disk-crunch issues like in the old architecture, manual moving of email accounts would be done. Concerns/doubts: I'm not convinced with the synchronously-replicated filesystem idea works well for heavy random/small-IO. GlusterFS isn't working for us yet, I'm not sure if there's another filesystem out there for this use case. The idea was to keep identical pairs and use DNS round-robin for email delivery and IMAP/POP3 access. And if one the servers went down for whatever reasons (planned/unplanned), we'd move the IP to the other server in the pair. In filesystems like Lustre, I get the advantage of a single namespace whereby I do not have to worry about manually migrating accounts around and updating MAILHOME paths and other metadata/data. Questions: What are the typical methods used to scale up/out with the traditional software (courier-imapd / dovecot)? Do traditional software that store on a locally mounted filesystem pose a roadblock to scale out with minimal "problems"? Does one have to re-write (parts of) these to work with an object-storage of some sort - such as OpenStack object storage?

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• apache/httpd responds slower under EL6.1 than EL5.6 (centos)

  - by daniel

  I've read through other threads on performance differences between RHEL6 and RHEL5, but none seem a tight match to mine. My issue manifests itself in slightly slower average response time (20ms) per request. I have about 10/10 servers of the same hardware spec with Cent6.1 and Cent5.6. The issue is consistent across the group. I am running Ruby on Rails with Passenger. Apache config is identical (checked out from the same SVN repo) Ruby and Passenger are identical builds. Application is identical and being served traffic round robin. mod_worker An interesting clue from server-status: The Cent6.1 servers have a steady 20-40 threads in the "Reading Request" state while the Cent5.6 servers have around 1. I'm graphing this so I can see it trend over time. I also have a bunch of much newer machines that are significantly faster and are running Cent6.1. They dust all the older machines in response time, but I can see they also have a steady 20-40 threads in the "Reading Request" state. This makes me believe I can get their response time down, if I can figure out what is holding up these requests. My gut is telling me that I need to tune some network setting in sysctl, but I haven't figured it out yet. Help is appreciated.

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• Developer’s Life – Every Developer is a Superman

  - by Pinal Dave

  I enjoyed comparing developers to Spiderman so much, that I have decided to continue the trend and encourage some of my favorite people (developers) with another favorite superhero – Superman.  Superman is probably the most famous superhero – and one of the most inspiring. Everyone has their own favorite, but Superman has been the longest enduring of all comic book characters.  Clark Kent has inspired multiple movie series, TV shows, books, cartoons, and costumes.  Superman’s enduring popularity has been attributed to his superhuman strength, integrity, dedication to good, and his humility in keeping his identity a secret. So how are developers like Superman? Well, read on my list of reasons. Secret Identities They have secret identities.  I’m not saying that all developers wear thick glasses and go by an alias like “Clark Kent.”  But developers certainly work in the background, making sure everything runs smoothly, often without recognition.  Like Superman, when they have done their job right, no one knows they were there. Working Alone You don’t have to work alone.  Superman doesn’t have a sidekick like Robin or Bat Girl, but he is a major player in the Justice League.  Developers have amazing skills, and they shouldn’t be afraid to unite those skills to solve some of the world’s major problems (like slow networks). Daily Inspiration Developers are inspiring.  Clark Kent works at The Daily Planet, Metropolis’ newspaper, which is lucky because he can keep some of the publicity Superman inspires under wraps.  Developers might go unnoticed sometimes, but when people hear about some of the tasks they accomplish on a daily basis, it inspires awe. Discover Your Superpowers You have to discover your superpowers.  Clark Kent didn’t just wake up one morning with the full understanding that he could fly, leap tall buildings in a single bound, and was stronger than a speeding locomotive.  He slowly discovered these powers (after a few comic book-worthy misunderstandings!).  Developers are always learning and growing as well.  You probably won’t wake up with super powers, either, but years of practice and continuing education can get you close. Every Day is a New Day The story continues.  The Superman comic books are still being printed, and have been in print since 1938.  There have been two TV series, (one, Smallville, was on TV for ten seasons) and multiple cartoon adaptations.  There have been multiple movies, with many different actors.  A new reboot came out last year, and another is set to premier in 2016.   So, developers, when you are having a bad day or a problem seems unsolvable – remember, the story will continue!  There is always tomorrow. I hope you are all enjoying reading about developers-as-superheroes as much as I am enjoying writing about them.  Please tell me how else developers are like Superheroes in the comments – especially if you know any developers who are faster than a speeding bullet and can leap tall buildings in a single bound. Reference: Pinal Dave (http://blog.sqlauthority.com)Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL Tagged: Developer, Superhero

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• Do We Indeed Have a Future? George Takei on Star Wars.

  - by Bil Simser

  George Takei (rhymes with Okay), probably best known for playing Hikaru Sulu on the original Star Trek, has always had deep concerns for the present and the future. Whether on Earth or among the stars, he has the welfare of humanity very much at heart. I was digging through my old copies of Famous Monsters of Filmland, a great publication on monster and films that I grew up with, and came across this. This was his reaction to STAR WARS from issue 139 of Famous Monsters of Filmland and was written June 6, 1977. It is reprinted here without permission but I hope since the message is still valid to this day and has never been reprinted anywhere, nobody will mind me sharing it. STAR WARS is the most pre-posterously diverting galactic escape and at the same time the most hideously credible portent of the future yet.While I thrilled to the exploits that reminded me of the heroics of Errol Flynn as Robin Hood, Burt Lancaster as the Crimson Pirate and Buster Crabbe as Flash Gordon, I was at the same time aghast at the phantasmagoric violence technology can place at our disposal. STAR WARS raised in my mind the question - do we indeed have a future?It seems to me what George Lucas has done is to masterfully guide us on a journey through space and time and bring us back face to face with today's reality. STAR WARS is more than science fiction, I think it is science fictitious reality.Just yesterday, June 7, 1977, I read that the United States will embark on the production of a neutron bomb - a bomb that will kill people on a gigantic scale but will not destroy buildings. A few days before that, I read that the Pentagon is fearful that the Soviets may have developed a warhead that could neutralize ours that have a capacity for that irrational concept overkill to the nth power. Already, it seems we have the technology to realize the awesome special effects simulations that we saw in the film.The political scene of STAR WARS is that of government by force and power, of revolutions based on some unfathomable grievance, survival through a combination of cunning and luck and success by the harnessing of technology -  a picture not very much at variance from the political headlines that we read today.And most of all, look at the people; both the heroes in the film and the reaction of the audience. First, the heroes; Luke Skywalker is a pretty but easily led youth. Without any real philosophy to guide him, he easily falls under the influence of a mystical old man believed previously to be an eccentric hermit. Recognize a 1960's hippie or a 1970's moonie? Han Solo has a philosophy coupled with courage and skill. His philosophy is money. His proficiency comes for a price - the highest. Solo is a thoroughly avaricious mercenary. And the Princess, a decisive, strong, self-confident and chilly woman. The audience cheered when she wielded a gun. In all three, I missed qualities that could be called humane - love, kindness, yes, I missed sensuality. I also missed a sense of ideals and faith. In this regard the machines seemed more human. They demonstrated real affection for each other and an occasional poutiness. They exhibited a sense of fidelity and constancy. The machines were humanized and the humans conversely seemed mechanical.As a member of the audience, I was swept up by the sheer romantic escapsim of it all. The deering-dos, the rope swing escape across the pit, the ray gun battles and especially the swash buckle with the ray swords. Great fun!But I just hope that we weren't too intoxicated by the escapism to be able to focus on the recognizable. I hope the beauty of the effects didn't narcotize our sensitivity to violence. I hope the people see through the fantastically well done futuristic mirrors to the disquieting reflection of our own society. I hope they enjoy STAR WARS without being "purely entertained".

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• Why some links appear in a new tab, others in a new window?

  - by SAMIR BHOGAYTA

  Originally, it was made to resolve problems on IE8 32 bits when you use a 32 bits OS. I changed "%ProgramFiles(x86)%" var, and now my issue is resolved for IE8 32 bits on my Windows 7 64 bits. Please try it, and tell me if everything work for you. For Win 7 64 bits : 1 - Create a new notepad document and paste this text : @echo off echo. echo IEREREG Version 1.07 for IE8 27.03.2009 echo by Kai Schaetzl http://iefaq.info echo installs and registers (if suitable) all DLLs known to be used by IE8. echo should only take a few seconds, but please be patient echo. REM ****************************** echo registering IE files REM IE files (= part of setup) regsvr32 /s /i browseui.dll REM regsvr32 /s /i browseui.dll,NI (unnecessary) regsvr32 /s corpol.dll regsvr32 /s dxtmsft.dll regsvr32 /s dxtrans.dll REM simple HTML Mail API regsvr32 /s "%ProgramFiles(x86)%\internet explorer\hmmapi.dll" REM group policy snap-in regsvr32 /s ieaksie.dll REM smart screen regsvr32 /s ieapfltr.dll REM ieak branding regsvr32 /s iedkcs32.dll REM dev tools regsvr32 /s "%ProgramFiles(x86)%\internet explorer\iedvtool.dll" regsvr32 /s iepeers.dll REM Symptom: IE8 closes immediately on launch, missing from IE7 regsvr32 /s "%ProgramFiles(x86)%\internet explorer\ieproxy.dll" REM no install point anymore REM regsvr32 /s /i iesetup.dll REM no reg point anymore REM regsvr32 /s imgutil.dll regsvr32 /s /i /n inetcpl.cpl REM no install point anymore REM regsvr32 /s /i inseng.dll regsvr32 /s jscript.dll REM license manager regsvr32 /s licmgr10.dll REM regsvr32 /s msapsspc.dll REM regsvr32 /s mshta.exe REM VS debugger regsvr32 /s msdbg2.dll REM no install point anymore REM regsvr32 /s /i mshtml.dll regsvr32 /s mshtmled.dll regsvr32 /s msident.dll REM no reg point anymore REM regsvr32 /s msrating.dll REM multimedia timer regsvr32 /s mstime.dll REM no install point anymore REM regsvr32 /s /i occache.dll REM process debug manager regsvr32 /s "%ProgramFiles(x86)%\internet explorer\pdm.dll" REM no reg point anymore REM regsvr32 /s pngfilt.dll REM regsvr32 /s /i setupwbv.dll (not there anymore!) regsvr32 /s tdc.ocx regsvr32 /s /i urlmon.dll REM regsvr32 /s /i urlmon.dll,NI,HKLM regsvr32 /s vbscript.dll REM VML renderer regsvr32 /s "%CommonProgramFiles%\microsoft shared\vgx\vgx.dll" REM no install point anymore REM regsvr32 /s /i webcheck.dll regsvr32 /s /i /n wininet.dll REM ****************************** echo registering system files REM additional system dlls known to be used by IE REM added 11.05.2006 Symptom: Add-Ons-Manager menu entry is present but nothing happens regsvr32 /s extmgr.dll REM added 12.05.2006 Symptom: Javascript links don't work (Robin Walker) .NET hub file regsvr32 /s mscoree.dll REM added 23.03.2009 Symptom: Find on this page is blank regsvr32 /s oleacc.dll REM added 24.03.2009 Symptom: Printing problems, open in new window regsvr32 /s ole32.dll REM mscorier.dll REM mscories.dll REM Symptom: open in new tab/window not working regsvr32 /s actxprxy.dll regsvr32 /s asctrls.ocx regsvr32 /s cdfview.dll regsvr32 /s comcat.dll regsvr32 /s /i /n comctl32.dll regsvr32 /s cryptdlg.dll regsvr32 /s /i /n digest.dll regsvr32 /s dispex.dll regsvr32 /s hlink.dll regsvr32 /s mlang.dll regsvr32 /s mobsync.dll regsvr32 /s /i msieftp.dll REM regsvr32 /s msnsspc.dll #no entry point regsvr32 /s msr2c.dll regsvr32 /s msxml.dll regsvr32 /s oleaut32.dll REM regsvr32 /s plugin.ocx #no entry point regsvr32 /s proctexe.ocx REM plus DllRegisterServerEx ExA ExW ... ? regsvr32 /s /i scrobj.dll REM shdocvw.dll hasn't been updated for IE7 and IE8, it still registers itself for the Windows Internet Controls regsvr32 /s /i shdocvw.dll regsvr32 /s sendmail.dll REM ****************************** REM PKI/crypto functionality REM initpki can take very long to run and is rarely a problem REM if there are problems with crypto, SSL, certificates REM remove the three following REMs from the lines REM echo We are almost done except one crypto file REM echo but this will take very long, be patient! REM regsvr32 /s /i:A initpki.dll REM ****************************** REM tabbed browser, do at the end, why originally with /n ? regsvr32 /s /i ieframe.dll REM ****************************** echo correcting bugs in the registry REM do some corrective work REM Symptom: new tabs page cannot display content because it cannot access the controls (added 27. 3.2009) REM This is a result of a bug in shdocvw.dll (see above), probably only on Windows XP reg add "HKCR\TypeLib\{EAB22AC0-30C1-11CF-A7EB-0000C05BAE0B}\1.1\0\win32" /ve /t REG_SZ /d %systemroot%\system32\ieframe.dll /f REM ****************************** echo all tasks have been finished echo. pause 2 - Close all your IE windows and processes. 3 - Save your document on your Desktop by example, with the .bat extension. Right-click on it, and select "Run as administrator". 4 - Test if this tip resolved your issue by openning IE. If you use a Windows 32 bits version, please replace %ProgramFiles(x86)% by %ProgramFiles% in your .bat file.

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• SQL Azure: Notes on Building a Shard Technology

  - by Herve Roggero

  In Chapter 10 of the book on SQL Azure (http://www.apress.com/book/view/9781430229612) I am co-authoring, I am digging deeper in what it takes to write a Shard. It's actually a pretty cool exercise, and I wanted to share some thoughts on how I am designing the technology. A Shard is a technology that spreads the load of database requests over multiple databases, as transparently as possible. The type of shard I am building is called a Vertical Partition Shard  (VPS). A VPS is a mechanism by which the data is stored in one or more databases behind the scenes, but your code has no idea at design time which data is in which database. It's like having a mini cloud for records instead of services. Imagine you have three SQL Azure databases that have the same schema (DB1, DB2 and DB3), you would like to issue a SELECT * FROM Users on all three databases, concatenate the results into a single resultset, and order by last name. Imagine you want to ensure your code doesn't need to change if you add a new database to the shard (DB4). Now imagine that you want to make sure all three databases are queried at the same time, in a multi-threaded manner so your code doesn't have to wait for three database calls sequentially. Then, imagine you would like to obtain a breadcrumb (in the form of a new, virtual column) that gives you a hint as to which database a record came from, so that you could update it if needed. Now imagine all that is done through the standard SqlClient library... and you have the Shard I am currently building. Here are some lessons learned and techniques I am using with this shard: Parellel Processing: Querying databases in parallel is not too hard using the Task Parallel Library; all you need is to lock your resources when needed Deleting/Updating Data: That's not too bad either as long as you have a breadcrumb. However it becomes more difficult if you need to update a single record and you don't know in which database it is. Inserting Data: I am using a round-robin approach in which each new insert request is directed to the next database in the shard. Not sure how to deal with Bulk Loads just yet... Shard Databases:  I use a static collection of SqlConnection objects which needs to be loaded once; from there on all the Shard commands use this collection Extension Methods: In order to make it look like the Shard commands are part of the SqlClient class I use extension methods. For example I added ExecuteShardQuery and ExecuteShardNonQuery methods to SqlClient. Exceptions: Capturing exceptions in a multi-threaded code is interesting... but I kept it simple for now. I am using the ConcurrentQueue to store my exceptions. Database GUID: Every database in the shard is given a GUID, which is calculated based on the connection string's values. DataTable. The Shard methods return a DataTable object which can be bound to objects.  I will be sharing the code soon as an open-source project in CodePlex. Please stay tuned on twitter to know when it will be available (@hroggero). Or check www.bluesyntax.net for updates on the shard. Thanks!

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• How the number of indexes built on a table can impact performances?

  - by Davide Mauri

  We all know that putting too many indexes (I’m talking of non-clustered index only, of course) on table may produce performance problems due to the overhead that each index bring to all insert/update/delete operations on that table. But how much? I mean, we all agree – I think – that, generally speaking, having many indexes on a table is “bad”. But how bad it can be? How much the performance will degrade? And on a concurrent system how much this situation can also hurts SELECT performances? If SQL Server take more time to update a row on a table due to the amount of indexes it also has to update, this also means that locks will be held for more time, slowing down the perceived performance of all queries involved. I was quite curious to measure this, also because when teaching it’s by far more impressive and effective to show to attended a chart with the measured impact, so that they can really “feel” what it means! To do the tests, I’ve create a script that creates a table (that has a clustered index on the primary key which is an identity column) , loads 1000 rows into the table (inserting 1000 row using only one insert, instead of issuing 1000 insert of one row, in order to minimize the overhead needed to handle the transaction, that would have otherwise ), and measures the time taken to do it. The process is then repeated 16 times, each time adding a new index on the table, using columns from table in a round-robin fashion. Test are done against different row sizes, so that it’s possible to check if performance changes depending on row size. The result are interesting, although expected. This is the chart showing how much time it takes to insert 1000 on a table that has from 0 to 16 non-clustered indexes. Each test has been run 20 times in order to have an average value. The value has been cleaned from outliers value due to unpredictable performance fluctuations due to machine activity. The test shows that in a  table with a row size of 80 bytes, 1000 rows can be inserted in 9,05 msec if no indexes are present on the table, and the value grows up to 88 (!!!) msec when you have 16 indexes on it This means a impact on performance of 975%. That’s *huge*! Now, what happens if we have a bigger row size? Say that we have a table with a row size of 1520 byte. Here’s the data, from 0 to 16 indexes on that table: In this case we need near 22 msec to insert 1000 in a table with no indexes, but we need more that 500msec if the table has 16 active indexes! Now we’re talking of a 2410% impact on performance! Now we can have a tangible idea of what’s the impact of having (too?) many indexes on a table and also how the size of a row also impact performances. That’s why the golden rule of OLTP databases “few indexes, but good” is so true! (And in fact last week I saw a database with tables with 1700bytes row size and 23 (!!!) indexes on them!) This also means that a too heavy denormalization is really not a good idea (we’re always talking about OLTP systems, keep it in mind), since the performance get worse with the increase of the row size. So, be careful out there, and keep in mind the “equilibrium” is the key world of a database professional: equilibrium between read and write performance, between normalization and denormalization, between to few and too may indexes. PS Tests are done on a VMWare Workstation 7 VM with 2 CPU and 4 GB of Memory. Host machine is a Dell Precsioni M6500 with i7 Extreme X920 Quad-Core HT 2.0Ghz and 16Gb of RAM. Database is stored on a SSD Intel X-25E Drive, Simple Recovery Model, running on SQL Server 2008 R2. If you also want to to tests on your own, you can download the test script here: Open TestIndexPerformance.sql

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