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• How to configure mod_proxy_balancer to gracefully fail under high load

  - by bramp

  We have a system which has one Apache instance in front of multiple tomcats. These tomcats then connect to various databases. We balance the load to the tomcat with mod_proxy_balancer. Currently we are receiving 100 requests a second, the load on the Apache server is quite low, but due to database heavy operations on the tomcats, the load there is roughly 25% (of what I estimate they can handle). In a few weeks there is an event happening and we estimate that our requests will jump significant, maybe by a factor of 10. I'm doing everything I can do reduce the load on our tomcats, but I know we are going to run out of capacity, so I would like to fail gracefully. By this I mean, instead of trying to deal with too many connections which all timeout, I would like Apache to somehow monitor average response time, and as soon as the response time to Tomcat is getting above some threshold, I would like a error page displayed. This means that users who are lucky still get a page rendered quickly, and those who are unlucky get a error page quickly. Instead of everyone waiting far too long for their page, and eventually everyone timing out, and the database being swamped with queries which are never used. Hopefully this makes sense, so I was looking for suggestions on how I could achieve this. thanks

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• Building a PC, advice on SSD/Hybrid Hard Drives

  - by Jamie Hartnoll

  I am looking at building a new PC, it's mainly for office (graphics heavy) use and programming. Looking for good performance with opening and closing programs and files as well as a fast boot. I plan to have 3 primary hard drives Windows 7 Programs (photoshop etc) Current Files (There'll also be a large storage capacity back up drive, but this will be the Seagate drive I already have.) So, my question is, looking at standard "old fashioned" hard drives and SSD drives, obviously there's a massive price difference. I have been looking at drives like this: http://www.ebuyer.com/268693-corsair-120gb-force-3-ssd-cssd-f120gb3-bk-cssd-f120gb3-bk and this: http://www.ebuyer.com/321969-momentus-xt-750gb-sata-2-5in-7200rpm-hybrid-8gb-ssd-in-st750lx003 Having no experience of using either I don't know what's the most efficient thing to go for. Clearly the SSD will have better performance, but: If, for example, I had an SSD for Windows (say about 100gB), that would clearly give me the boot speed I want, then I guess my real questions are: If I were to buy one more SSD, would it give the greatest improvement on standard performance if used to store programs, or currently used files? Given that the OS is on an SSD, should I not bother with the 3 drives and instead, partition that Hybrid drive to store programs and currently used files on it? Obviously, option two is cheaper and option one could cause me storage issues, but that's when I can dump files I am not currently using onto another drive. Any, I am open to suggestions... so what do you suggest?!

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• Windows7 shows a drive as full in summary but files, including backup folder, shown on drive are ver

  - by Rob

  I have a drive partitioned so it is seen by Windows as 2 drives: C:\ and D:\ Windows7 shows D:\ as full up in the graphical summary in 'My Computer' summary of all the drives, e.g. the bar graph indicates full and nearly all of the drive's capacity, 108Gb, is full. So I go into the D:\ drive to look at the files, I see several folders. I select them all and the right click menu Properties to count their size, expecting the value to be about the same as what Windows reports in the summary, i.e. nearly 108Gb. But the properties shows the files are very small, Kbs and Mbs, nowhere near 108Gbs. One of the folders is a backup, but its size is very small. I've checked the folder options to show all system files and hidden files too - and counted these in the properties. Something invisible is holding the space. What is happening here? I'm afraid to delete anything if it removes valuable backups. Have I got huge backups here? Why can't I see them? How do I see them?

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• filter / directing URLs coming onto a network

  - by Jon

  Hi all, I an not sure if this is possible or not but what i would like to do is as follows: I have one IP address (dynamic using zoneedit.com to keep it upto date). I have one webserver running my main site which is an Ubuntu machine running Apache. I also have a windows 2008 server running another site. Just to confuse things I also run part of my Apache site on the windows server, currently using proxypassreverse to get the information from it. So it looks something like this: IP 1.2.3.4 maps to mydomain.com as well as myotherdomain.com All requests that come into port 80 are forwarded to the Apache box and I use Virtualhost settings to proxy the windows sites where needed. so mydomain.com is an Apache site mydomain.com/mywindowssection is the Apache server using proxypassreverse to get part of the site from the Windows server myotherdomain.com uses Apache and proxypassreverse to get the whole site. What I would like to be able to do is forward all http requests that come into my network to one machine that figures out who should be serving that content. so: mydomain.com would go to the Apache machine myotherdomain.com would go the windows machine. I am just in the process of setting up an Astaro gateway (never done this before so taking a while to configure) as my firewall, dns, dhcp etc, don't know if this can handle it. I have the capacity to run a VM on the network if a seperate box would be needed for this process as well. Thanks for any and all feedback. Jon

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• R glm standard error estimate differences to SAS PROC GENMOD

  - by Michelle

  I am converting a SAS PROC GENMOD example into R, using glm in R. The SAS code was: proc genmod data=data0 namelen=30; model boxcoxy=boxcoxxy ~ AGEGRP4 + AGEGRP5 + AGEGRP6 + AGEGRP7 + AGEGRP8 + RACE1 + RACE3 + WEEKEND + SEQ/dist=normal; FREQ REPLICATE_VAR; run; My R code is: parmsg2 <- glm(boxcoxxy ~ AGEGRP4 + AGEGRP5 + AGEGRP6 + AGEGRP7 + AGEGRP8 + RACE1 + RACE3 + WEEKEND + SEQ , data=data0, family=gaussian, weights = REPLICATE_VAR) When I use summary(parmsg2) I get the same coefficient estimates as in SAS, but my standard errors are wildly different. The summary output from SAS is: Name df Estimate StdErr LowerWaldCL UpperWaldCL ChiSq ProbChiSq Intercept 1 6.5007436 .00078884 6.4991975 6.5022897 67911982 0 agegrp4 1 .64607262 .00105425 .64400633 .64813891 375556.79 0 agegrp5 1 .4191395 .00089722 .41738099 .42089802 218233.76 0 agegrp6 1 -.22518765 .00083118 -.22681672 -.22355857 73401.113 0 agegrp7 1 -1.7445189 .00087569 -1.7462352 -1.7428026 3968762.2 0 agegrp8 1 -2.2908855 .00109766 -2.2930369 -2.2887342 4355849.4 0 race1 1 -.13454883 .00080672 -.13612997 -.13296769 27817.29 0 race3 1 -.20607036 .00070966 -.20746127 -.20467944 84319.131 0 weekend 1 .0327884 .00044731 .0319117 .03366511 5373.1931 0 seq2 1 -.47509583 .00047337 -.47602363 -.47416804 1007291.3 0 Scale 1 2.9328613 .00015586 2.9325559 2.9331668 -127 The summary output from R is: Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept) 6.50074 0.10354 62.785 < 2e-16 AGEGRP4 0.64607 0.13838 4.669 3.07e-06 AGEGRP5 0.41914 0.11776 3.559 0.000374 AGEGRP6 -0.22519 0.10910 -2.064 0.039031 AGEGRP7 -1.74452 0.11494 -15.178 < 2e-16 AGEGRP8 -2.29089 0.14407 -15.901 < 2e-16 RACE1 -0.13455 0.10589 -1.271 0.203865 RACE3 -0.20607 0.09315 -2.212 0.026967 WEEKEND 0.03279 0.05871 0.558 0.576535 SEQ -0.47510 0.06213 -7.646 2.25e-14 The importance of the difference in the standard errors is that the SAS coefficients are all statistically significant, but the RACE1 and WEEKEND coefficients in the R output are not. I have found a formula to calculate the Wald confidence intervals in R, but this is pointless given the difference in the standard errors, as I will not get the same results. Apparently SAS uses a ridge-stabilized Newton-Raphson algorithm for its estimates, which are ML. The information I read about the glm function in R is that the results should be equivalent to ML. What can I do to change my estimation procedure in R so that I get the equivalent coefficents and standard error estimates that were produced in SAS? To update, thanks to Spacedman's answer, I used weights because the data are from individuals in a dietary survey, and REPLICATE_VAR is a balanced repeated replication weight, that is an integer (and quite large, in the order of 1000s or 10000s). The website that describes the weight is here. I don't know why the FREQ rather than the WEIGHT command was used in SAS. I will now test by expanding the number of observations using REPLICATE_VAR and rerunning the analysis.

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• HTTP Error: 413 Request Entity Too Large

  - by Torben Gundtofte-Bruun

  What I have: I have an iPhone app that sends HTTP POST requests (XML format) to a web service written in PHP. This is on a hosted virtual private server so I can edit httpd.conf and other files on the server, and restart Apache. The problem: The web service works perfectly as long as the request is not too large, but around 1MB is the limit. After that, the server responds with: <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <html><head> <title>413 Request Entity Too Large</title> </head><body> <h1>Request Entity Too Large</h1> The requested resource<br />/<br /> does not allow request data with POST requests, or the amount of data provided in the request exceeds the capacity limit. </body></html> The web service writes its own log file, and I can see that small messages are processed fine. Larger messages are not logged at all so I guess that something in Apache rejects them before they even reach the web service? Things I've tried without success: (I've restarted Apache after every change. These steps are incremental.) hosting provider's web-based configuration panel: disable mod_security httpd.conf: LimitXMLRequestBody 0 and LimitRequestBody 0 httpd.conf: LimitXMLRequestBody 100000000 and LimitRequestBody 100000000 httpd.conf: SecRequestBodyLimit 100000000 At this stage, Apache's error.log contains a message: ModSecurity: Request body no files data length is larger than the configured limit (1048576) It looks like my step #4 didn't really take, which is consistent with step #1 but does not explain why mod_security appears to be active after all. What more can I try, to get the web service to receive large messages?

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• Replication keeps popping up on SharePoint databases

  - by Ddono25

  My typical discovery scenario: We receive an alert that the transaction log is growing quickly. We are in Simple Recovery so I go to check it out. Log is already sized to 100GB and is at 80% capacity. I run the "Whats using my log files" script from SQL Server Central and see that Replication is enabled on the database. We do not set up replication, and I don't think Replication can be done on SharePoint content db's as Replication is not supported (requires PK on all tables). This has been occurring on random servers (about 5 so far, all within the past three weeks) and it only occurs on Content Databases. sp_removedbreplication does not always work in removing the Replication either. We have found that we need to run the sp_removedbreplication, change all db owners to SA and reset Recovery Mode to Simple to completely eradicate any vestiges of this bug. How would Replication be enabling itself? We have never set up Replication on these servers. There is no evidence of any type of Replication other than the 'log_reuse_wait_desc' from the DMV query and log growth. Any help on this ghost would be appreciated!

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• OpenVPN Server - CPU is pegged out

  - by ericl42

  Hello, I am configuring OpenVPN to act as a SSL tunnel for a remote location. I have OpenVPN1 at our current location acting as a server then OpenVPN2 at the other location that is acting as a client but is also acting as a DHCP server to machines behind it so they are basically connected to the local LAN. Everything is set up fine and I can talk from location A to location B with no problems like everyone is local. I am however having some performance issues. OpenVPN1 CPU is pegged to 100% the entire time I am copying or doing any type of activity through the tunnel. I expect some CPU usage going up but nothing like this. It's really killing my performance. OpenVPN1 is running in ESX right now with 2 gig RAM and 4 procs with unlimited bursting capacity. I am using AES-192 encryption with a 1024 key. Any idea how I can get my CPU down on OpenVPN1 and my download/upload speeds higher between the tunnel? Thanks. edit: Turning down the logging helped boost the throughput a little bit, but I am still fairly shy of where I believe I should be. Also I am still maxed out on the CPU. Does anyone have any ideas? I am really stuck on this. Thanks.

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• Is .htaccess slowing down my dedicated server?

  - by David Robles

  First of all, I consider myself more a programmer than a servers guy. I have a website where I receive about 3,000 visits per day, which I think is a lot less than the max capacity for a dedicated server. However, I've noticed that the connection to the website is pretty slow, e.g., to load images, to connect to it via SSH, etc. I configured .httaccess recently to avoid hotlinking to images in my server (i.e. .jpg, .gif and .png), and I was wondering if that could be slowing down my website. This is the configuration that I have: # BEGIN WordPress <IfModule mod_rewrite.c> RewriteEngine On RewriteBase / RewriteEngine on RewriteCond %{HTTP_REFERER} !^$ RewriteCond %{HTTP_REFERER} !^http://www.mysite.com/.*$ [NC] RewriteCond %{HTTP_REFERER} !^http://www.mysite.com$ [NC] RewriteRule .*\.(jpg|jpeg|gif|png|bmp|swf)$ http://www.google.com/ [R,NC] RewriteCond %{REQUEST_FILENAME} !-f RewriteCond %{REQUEST_FILENAME} !-d RewriteRule . /index.php [L] </IfModule> # END WordPress I found some code to do that in google, and I just copied to .htacces since I'm not an expert in apache. It works, but I don't know if that is the best way to do it. How can I see if that is the reason why the server is slow? Are there any tools to monitor it? What would you do guys? Thanks in advance!

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• File system concepts (df command)

  - by mkab

  I'm finding it difficult to understand some stuffs about the df command. Suppose I type df and I have the following output Filesystem 1k-blocks Used Avail Capacity Mounted on /dev/da0s1 some number some number number percentage /win /dev/da0s2 some number some number number percentage /win/home /dev/da0s3a some number some number number percentage / devfs some number some number number percentage /dev /dev/da0s3g some number some number number percentage /local /dev/da0s3h some number some number -number 102% /reste /dev/da0s3d some number some number number percentage /tmp /dev/da1s3f some number some number number percentage /usr /dev/da1s3e some number some number number percentage /var /dev/da1s1a some number some number number percentage /public Are the answers to the following questions correct? How many physical drives do I have? Ans: 2. da0s1 and da1s1 How many physical partitions on each disk? Ans: 8 for da0s1 and 1 for da1s1 How many BSD partition on each physical partition Ans: Impossible to determine. We have to use the -T to determine its type How is it possible for the file system /dev/da0s3h filled at 102%? And where is this overflowed data written?Ans: I have no idea for this one Thanks.

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• Network card very slow, only on Windows

  - by J Penguin

  This only happens to 1 of my machine, and only when booting into Windows 7. No matter what network card I put in, Windows would default its mode to 10Mbps full duplex. Transfer speed is approximately 1 MB/s. If I set it to 100Mbps, the transfer drops to 100-200K/s. If I set it to 1000Mbps, the connection is lost completely. I've tried swapping in different cards, both PCI-E and PCI. I'v etried update the windows, I've tried reinstalling the drivers... On this very same machine, if I boot into Fedora, it can use the card at its full capacity 1000Mbps transfering 80+ MB/s And all the cards work just fine when plugging into other machines on the same network. I'm very curious. What could be the reason for this? The only different software that this machine has is virtual box with a VPN emulator, but disabling that VPN doesn't seem to do anything. I would like to get this fixed, hopefully, without reinstalling windows _< Will that be possible?

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• Function Point Analysis -- a seriously over-estimating technique?

  - by kizzx2

  I know questions about FPA has been asked numerous times before, but this time I'm taking a more analytical angle at it, backed up with data. 1. First, some data This question is based on a tutorial. He had a "Sample Count" section where he demonstrated it step by step. You can see some screenshots of his sample application here. In the end, he calculated the unadjusted FP to be 99. There is another article on InformIT with industry data on typical hour/FP. It ranges from 2 hours/FP to 27.4 hours/FP. Let's try to stick with 2 for the moment (since SO readers are probably the more efficient crowd :p). 2. Reality check!? Now just check out the screenshots again. Do a little math here 99 * 2 = 198 hours 198 hours / 40 hours per week = 5 weeks Seriously? That sample application is going to take 5 weeks to implement? Is it just my feeling that it wouldn't take any decent programmer longer than one week to have it completed? Now let's try estimating the cost of the project. We'll use New York's minimum wage at the moment (Wikipedia), which is $7.25 198 * 7.25 = $1435.5 From what I could see from the screenshots, this application is a small excel-improvement app. I could have bought MS Office Pro for 200 bucks which gives me greater interoperability (.xls files) and flexibility (spreadsheets). (For the record, that same Web site has another article discussing productivity. It seems like they typically use 4.2 hours/FP, which gives us even more shocking stats: 99 * 4.2 = 415 hours = 10 weeks = almost 3 whopping months! 415 hours * $7.25 = $3000 zomg (That's even assuming that all our poor coders get the minimum wage!) 3. Am I missing something here? Right now, I could come up with several possible explanation: FPA is really only suited for bigger projects (1000+ FPs) so it becomes extremely inaccurate at smaller scale. The hours/FP metric fluctuates abruptly from team to team, project to project. For a small project like this, we could have used something like 0.5 hour/FP or something. (Now this kind of makes the whole estimation thing pointless, unless my firm does the same type of projects for several years with the same team, not really common.) From my experience with several software metrics, Function Point is really not a lightweight metric. If the hour/FP thing fluctuates so much, then what's the point, maybe I could have gone with User Story Points which is a lot faster to get and arguably almost as uncertain. What would be the FP experts' answers to this?

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• How to configure mod_proxy_balancer to gracefully fail under high load

  - by bramp

  We have a system which has one Apache instance in front of multiple tomcats. These tomcats then connect to various databases. We balance the load to the tomcat with mod_proxy_balancer. Currently we are receiving 100 requests a second, the load on the Apache server is quite low, but due to database heavy operations on the tomcats, the load there is roughly 25% (of what I estimate they can handle). In a few weeks there is an event happening and we estimate that our requests will jump significant, maybe by a factor of 10. I'm doing everything I can do reduce the load on our tomcats, but I know we are going to run out of capacity, so I would like to fail gracefully. By this I mean, instead of trying to deal with too many connections which all timeout, I would like Apache to somehow monitor average response time, and as soon as the response time to Tomcat is getting above some threshold, I would like a error page displayed. This means that users who are lucky still get a page rendered quickly, and those who are unlucky get a error page quickly. Instead of everyone waiting far too long for their page, and eventually everyone timing out, and the database being swamped with queries which are never used. Hopefully this makes sense, so I was looking for suggestions on how I could achieve this. thanks

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• DAS vs SAN storage for serving 2 to 4 nodes

  - by Luke404

  We currently have 4 Linux nodes with local storage, arranged in two active/passive pairs with storage mirrored using DRBD, running virtual machines (actually using Xen Hypervisor) for typical hosting workloads (mail, web, a couple VPS, etc.). We're approaching the (presumed) maximum IOPS of those servers, and we're planning to migrate to an external storage solution with two active nodes, with capacity for up to four active nodes. Since we're an all-Dell shop I've done some research and found the MD3200 / MD3200i products should be the ones we're looking for. We are pretty sure we won't be attaching more than 4 hosts on a single storage and I'm wondering if there is any clear advantage for one or the other. In theory I should be able to attach 4 SAS hosts to a single MD3200 (single links on a single controller MD3200, or dual redundant SAS links from each host to a dual-controller MD3200), or 4 iSCSI hosts to a single MD3200i (directly on its 4 GigE ports without any switch, again with dual links for the dual controller option). Both setups should let us implement live VM migration since all hosts can access all the LUNs at the same time, and also some shared filesystem like GFS2 or OCFS2. Also, both setups should allow full redundancy of the whole system (assuming dual controllers in the storage). One difference I can see is that the DAS solution is actually limited to 4 hosts while the iSCSI one should be able to grow to more hosts (adding two GigE switches to the mix). One point for the iSCSI solution is that it would allow us to start out with our current nodes and upgrade them at a later time (we can't add other SAS controllers, but they already have 4 GigE ports each). With the right (iSCSI|SAS) controllers I should be able to connect diskless nodes and boot them off the external storage which I think is a good thing (get rid of any local storage). On the other hand, I would have thought the SAS one to be cheaper but it seems like an MD3200 actually costs a little less than an MD3200i (?) (please note: I've used Dell gear in my examples since that's what we're looking for but I assume the same goes with other vendors) I would like to know if my assumptions above are correct, and if I'm missing any important difference between the two setups.

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• What is the replacement of the floppy

  - by alexanderpas

  While CD (and to an lesser extend DVD) disks have reached the price-point of the floppy, they have one significant downside, it is WORM (Write-Once Read-Many) media, allowing it to be used only one single time, and you need to be explicit in writing the data to the actual media (you need to burn it.) While CD-RW solves the "use only once" problem, it is still EWORM (Erasable Write-Once Read-Many) media, which still means you need to be explicit in writing the data to the actual media (you still need to burn it.), and also, you still need to be very explicit in erasing it. (simple delete is not possible.) Okay, we can use a CD-RW in Packet Writing mode, however the downside to that, is that this mode is not very universal, and also, not the native mode of the media. Now, while USB-sticks and SD-cards may not have the poblems of the CD, they have a whole other kind of problem: their PRICE! USB-sticks and SD cards are generally 10 to 100 times as expensive as diskettes per piece. SD-cards, in addition have an added problem, because they need a reader to operate. While it is a very standard thing, it is not default equipment on the computer like the CD drive or USB port (or historically the diskette drive). You wouldn't give out an USB stick or SD card with a 100 kB text file, not caring weither you would get it back or not. So, to recap: CD & DVD are basically WORM media. SD cards and USB sticks are relatively expensive. SD cards also needs special readers. Diskettes have a very low data-rate Diskettes have a very low storage capacity. Now, is there a media out there that solves all these problems, or is there a way to get (very) small USB sticks or SD cards for a very low price (as they're the closest thing to diskette).

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• What is the best way/Software to manage multiple short lived instances of virtual machines ?

  - by Newtopian

  Hi, We have a QA department that have to test our software on multiple combination of OS and DMBS. With Windows spewing out many different versions the combinatorial math of all this can be daunting. So we decided on visualizing our setups but so far it only displaces the problem. The cost of hardware is expensive and we need many different combination far exceeding your server capacity to deliver. Also, these instances are throw away, once the test is complete we no longer need it, furthermore to ensure proper test isolation we should start fresh from a new instance. Lastly we only need a small subset of these system online at any given time. What I am looking for is a way to manage inventory so that our QA staff can order instances to be put online as required and discarded once used. Instances are spawned from a pool of freshly installed systems with the appropriate combination ready to accept our software. It also should be possible for two or more people to start the same instance at the same time, though we could manage without this if it proves too complex to put in place. Finally our budget is pretty thin, we can probably make some purchases but ideally expenditures should be kept to a minimum. To summarize we should be able to : Bring instances online on demand. Ideally should offer queue and scheduling management Destroy instances on demand Keep masters in inventory but not online. Manage large inventory of VMs (30-100 maybe more) with small staff of users (5-10). Allow adding, deleting and changing instances from inventory (bring online, make changes and check back in, or create new and check in). Allow few long lived instances for support tools (normal VM server usage) Thanks for your answers

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• Windows 7 mapped drive kicking off OS X users

  - by Collin White

  I've mapped a network drive on my Windows 7 PC at my office. The windows machine has a few TB of storage that is being accessed by my development team (all running mac os 10.7). The share seems to work fine for a little while but will timeout and kick the mac users off and sometimes disallows a connection on the next attempt. Restarting the windows machine fixes the problem. I've tried this tutorial as well as setting the maximum session length in the Local Security Policy section to 99999 (I discovered 0 did not mean unlimited, only a 'reasonable ammount of time') anyway, the setting is now for ~208 days which is sufficient (see attached). I'm having trouble debugging this in general so if anyone has some pointers I'm all ears. This is a intermittent issue which in my opinion are the hardest kinds to debug. If anyone knows of how I might monitor connections from the PC that would also be pretty cool. Previously the files were hosted on a mac mini and everything was working just fine (the mini just didn't have the ability for the storage capacity we needed) so I believe it is some windows setting that is kicking users off. Anyway, thanks for reading.

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• Can my loop be optimized any more? (C++)

  - by Sagekilla

  Below is one of my inner loops that's run several thousand times, with input sizes of 20 - 1000 or more. Is there anything I can do to help squeeze any more performance out of this? I'm not looking to move this code to something like using tree codes (Barnes-Hut), but towards optimizing the actual calculations happening inside, since the same calculations occur in the Barnes-Hut algorithm. Any help is appreciated! typedef double real; struct Particle { Vector pos, vel, acc, jerk; Vector oldPos, oldVel, oldAcc, oldJerk; real mass; }; class Vector { private: real vec[3]; public: // Operators defined here }; real Gravity::interact(Particle *p, size_t numParticles) { PROFILE_FUNC(); real tau_q = 1e300; for (size_t i = 0; i < numParticles; i++) { p[i].jerk = 0; p[i].acc = 0; } for (size_t i = 0; i < numParticles; i++) { for (size_t j = i+1; j < numParticles; j++) { Vector r = p[j].pos - p[i].pos; Vector v = p[j].vel - p[i].vel; real r2 = lengthsq(r); real v2 = lengthsq(v); // Calculate inverse of |r|^3 real r3i = Constants::G * pow(r2, -1.5); // da = r / |r|^3 // dj = (v / |r|^3 - 3 * (r . v) * r / |r|^5 Vector da = r * r3i; Vector dj = (v - r * (3 * dot(r, v) / r2)) * r3i; // Calculate new acceleration and jerk p[i].acc += da * p[j].mass; p[i].jerk += dj * p[j].mass; p[j].acc -= da * p[i].mass; p[j].jerk -= dj * p[i].mass; // Collision estimation // Metric 1) tau = |r|^2 / |a(j) - a(i)| // Metric 2) tau = |r|^4 / |v|^4 real mij = p[i].mass + p[j].mass; real tau_est_q1 = r2 / (lengthsq(da) * mij * mij); real tau_est_q2 = (r2*r2) / (v2*v2); if (tau_est_q1 < tau_q) tau_q = tau_est_q1; if (tau_est_q2 < tau_q) tau_q = tau_est_q2; } } return sqrt(sqrt(tau_q)); }

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• Laptop battery holds charge, but won't charge any more.

  - by Jeff

  Ok, I'm sure I will need to replace either my battery or my AC adapter, but would rather not buy one if the other is the problem. My problem is. I have a Sager laptop that gets quite a bit of use. The charging has always been a little bit odd. If I was in the process of using it, it would charge just fine and stay On AC power. If I left it alone, however(power settings to ONLY turn off the monitor) in either Ubuntu or Windows 7 it decides that it didn't want to use AC power anymore and would just start draining the battery until it died. Now, suddenly, it won't charge at all. The capacity was great up to this point which happened in an instant. It will recognize the battery but won't see the AC power if plugged in while the battery is in. I can power up the laptop without the battery and it works fine. If I plug in the battery while powered up it will claim it's charging it, but it stays at the same percentage. If I unplug the power, it will switch over to Battery fine, but I have to power down and unplug the battery to get it back on AC power. I've had dying/dead batteries before but they typically won't hold a full charge anymore but it still winds up to 100% then drops quickly when unplugged. This seems more like a chip problem in the battery to me, but I'm not sure. Any ideas?

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• Determining the Source of a Given File System Mount on Unix [migrated]

  - by phobos51594

  Background Recently I have run into a bit of a snag on my home FreeBSD server. I recently upgraded it to the latest stable release, and I have noticed some strange behavior with the /var partition. Originally, I had the system configured such that /var had its own partition with /var/run and /var/log in memory disks (/tmp, too). After the upgrade, I notice there is a new, fourth memory disk mounting directly to /var that I had not set up manually and is not in my fstab. It is only 28 megs or so in size and is causing problems when trying to update my ports collection. The ramdisk mounts atuomagically at boot and cannot be unmounted while in multi-user mode. If I drop to single user mode, I am able to unmount it without issue, however rebooting causes it to pop right back up. System specifications have been included at the end of the post. Question Is there any way to determine exactly what is mounting a given memory disk (or any filesystem, for that matter) after it has been mounted? Alternately, does anybody have any ideas what might have caused the new /var ramdisk to pop up? System Specification # uname -a FreeBSD sarge 9.1-PRERELEASE FreeBSD 9.1-PRERELEASE #0: Thu Nov 22 14:02:13 PST 2012 donut@sarge:/usr/obj/usr/src/sys/GENERIC i386 # df Filesystem 1K-blocks Used Avail Capacity Mounted on /dev/da0s1a 515612 410728 63636 87% / devfs 1 1 0 100% /dev /dev/da0s1d 515612 287616 186748 61% /var /dev/da0s1e 6667808 2292824 3841560 37% /usr /dev/md0 63004 32 57932 0% /tmp /dev/md1 3484 8 3200 0% /var/run /dev/md2 31260 8 28752 0% /var/log /dev/md3 31260 512 28248 2% /var <-- This # cat /etc/fstab # Device Mountpoint FStype Options Dump Pass# /dev/da0s1a / ufs rw,noatime 1 1 /dev/da0s1d /var ufs rw,noatime 2 2 /dev/da0s1e /usr ufs rw,noatime 2 2 md /tmp mfs rw,-s64M,noatime 0 0 md /var/run mfs rw,-s4M,noatime 0 0 md /var/log mfs rw,-s32M,noatime 0 0 Thank you in advance for any assistance.

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• VMWare Setup with 2 Servers and a DAS (DELL MD3220)

  - by Kumala

  I am planning to use a VMWare based setup consisting of two VMWare servers (2 CPU, 256GB Memory) and a DAS (DELL MD3220 with 24x900GB disks). The virtual machines will be half running MS SQL databases (Application, Sharepoint, BI) and the other half of the VM will be file services, IIS. To enhance the capacity of the storage, we'll be adding a MD1220 enclosure with another 24x900GB to the MD3220. Both DAS will have 2 controllers. Our current measured IOPS is 1000 IOPS average, 7000 IOPS peak (those happen maybe twice per hour). We are in the planning phase now and are looking at the proper setup of the disks. The intention is to setup up both DAS one of the DAS with RAID 10 only and the other DAS with RAID 5. That will allow us to put the applications on the DAS that supports the application performance needs best. Question is how best to partition the two DASs to get best possible IOPS/MBps, each DAS will have to have 2 hot spares? For the RAID 5 Setup: Generally speaking, would it be better to have one single disk group across all 22 disks (24 - 2 hot spares) with both controllers assigned to the one disk group or is it better to have 2 disk groups each 11 disks, assigned to one of the two controllers? Same question for the RAID 10 setup: The plan is: 2 disks for logs (Raid 1), 2 Hotspare and 20 disks for RAID 10. Option 1: 5 * 4 disks (RAID 10), with two groups assigned to 1 controller and 3 groups to the other controller Option 2: One large RAID 10 across all the disks and have both controllers assigned to the same group? I would assume that there is no right or wrong, but it all depends very much on the specific application behaviour, so I am looking for some general ideas what the pros and cons are of the different options. IF there are other meaningful options, feel free to propose them.

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• Why might my Fedora 15 live USB persistent storage not work?

  - by Richard J Foster

  I created a Fedora 15 "live" USB stick using the live USB creator found at https://fedorahosted.org/liveusb-creator/ and the Fedora 15 i686 Desktop ISO image with the persistent storage space set to 4096MB. (The USB stick I have available has an 8GB capacity, so there should be plenty of space.) Fedora appears to boot correctly, however it seems that the persistent storage is not working. To verify this, I opened a terminal prompt, then did su - followed by yum update yum. As expected, I was informed that a new version was available. (The live CD contains version 3.2.29-4, at the time of typing 3.2.29-6 is the current version). After installing, I verified that the new version was installed by typing yum --version. I then shutdown the system using shutdown now. After the system had shut down, I rebooted and returned to the terminal prompt. On typing yum --version, I was informed that the version was 3.2.29-4 (i.e. the original version). Why might the persistent storage not be working? Is there anything I can do to fix it?

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• will heavy network traffic affect other connections on HP ProCurve V1810-48G?

  - by nn4l

  I have a HP ProCurve V1810-48G switch with a few servers connected to it (everything in one rack). The switch is practically in its default configuration. During copying of a few hundred GByte of data from server_a to server_b (using tar cf - data | ssh server_b 'cd myhome; tar xf -'), essentially saturating the network capacity between those two servers, I noticed network related error messages on the console of server_c - as if server_c is no longer able to send/receive traffic to server_d. After canceling the copy command everything was normal again. I would understand this if the network connection would use a shared resource, for example if server_a and server_c are in one datacenter, server_b and server_d are in another datacenter and both datacenters are connected with a 100 MBit line. But all of the mentioned servers are connected to the same switch and are located in the same IP network. I always thought that a connection between two servers on one switch will not affect any other server connected to the switch. It is also possible that the network related error messages are caused by something else - but I can't risk a network problem for any other system on this switch. Please advise.

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• Runge-Kutta Method with adaptive step

  - by infoholic_anonymous

  I am implementing Runge-Kutta method with adaptive step in matlab. I get different results as compared to matlab's own ode45 and my own implementation of Runge-Kutta method with fixed step. What am I doing wrong in my code? Is it possible? function [ result ] = rk4_modh( f, int, init, h, h_min ) % % f - function handle % int - interval - pair (x_min, x_max) % init - initial conditions - pair (y1(0),y2(0)) % h_min - lower limit for h (step length) % h - initial step length % x - independent variable ( for example time ) % y - dependent variable - vertical vector - in our case ( y1, y2 ) function [ k1, k2, k3, k4, ka, y ] = iteration( f, h, x, y ) % core functionality performed within loop k1 = h * f(x,y); k2 = h * f(x+h/2, y+k1/2); k3 = h * f(x+h/2, y+k2/2); k4 = h * f(x+h, y+k3); ka = (k1 + 2*k2 + 2*k3 + k4)/6; y = y + ka; end % constants % relative error eW = 1e-10; % absolute error eB = 1e-10; s = 0.9; b = 5; % initialization i = 1; x = int(1); y = init; while true hy = y; hx = x; %algorithm [ k1, k2, k3, k4, ka, y ] = iteration( f, h, x, y ); % error estimation for j=1:2 [ hk1, hk2, hk3, hk4, hka, hy ] = iteration( f, h/2, hx, hy ); hx = hx + h/2; end err(:,i) = abs(hy - y); % step adjustment e = abs( hy ) * eW + eB; a = min( e ./ err(:,i) )^(0.2); mul = a * s; if mul >= 1 % step length admitted keepH(i) = h; k(:,:,i) = [ k1, k2, k3, k4, ka ]; previous(i,:) = [ x+h, y' ]; %' i = i + 1; if floor( x + h + eB ) == int(2) break; else h = min( [mul*h, b*h, int(2)-x] ); x = x + keepH(i-1); end else % step length requires further adjustments h = mul * h; if ( h < h_min ) error('Computation with given precision impossible'); end end end result = struct( 'val', previous, 'k', k, 'err', err, 'h', keepH ); end The function in question is: function [ res ] = fun( x, y ) % res(1) = y(2) + y(1) * ( 0.9 - y(1)^2 - y(2)^2 ); res(2) = -y(1) + y(2) * ( 0.9 - y(1)^2 - y(2)^2 ); res = res'; %' end The call is: res = rk4( @fun, [0,20], [0.001; 0.001], 0.008 ); The resulting plot for x1 : The result of ode45( @fun, [0, 20], [0.001, 0.001] ) is:

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• What is the peak theoretical WiFi G user density? [closed]

  - by Bigbio2002

  I've seen a few WiFi capacity planning questions, and this one is related, but hopefully different enough not to be closed. Also, this is related specifically to 802.11g, but a similar question could be made for N. In order to squeeze more WiFi users into a space, the transmit power on the APs need to be reduced and the APs squeezed closer together. My question is, how far can you practically take this before the network becomes unusable? There will come a point where the transmit power is so weak that nobody will actually be able to pick up a connection, or be constantly roaming to/from APs spaced a few feet apart as they walk around. There are also only 3 available channels to use as well, which is a factor to consider. After determining the peak AP density, then multiply by users-per-AP, which should be easier to find out. After factoring all of this in and running some back-of-the-envelope calculations, I'd like to be able to get a figure of "XX users per 10ft^2" or something. This can be considered the physical limit of WiFi, and will keep people from asking about getting 3,000 people in a ballroom conference on WiFi. Can anyone with WiFi experience chime in, or better yet, provide some calculations for a more accurate figure? Assumptions: Let's assume an ideal environment with no reflection (think of a big, square, open room, with the APs spaced out on a plane), APs are placed on the ceiling so humans won't absorb the waves, and the only interference are from the APs themselves and the devices. As for what devices specifically, that's irrelevant for the first point of the question (AP density, so only channel and transmit power should matter). User experience: Wikipedia states that Wireless G has about 22Mbps maximum effective throughput, or about 2.75MB/s. For the purpose of this question, anything below 100KB/s per user can be deemed to be a poor user experience. As for roaming, I'll assume the user is standing in the same place, so hopefully that will be a non-issue.

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