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• mounting external hard drive EXT4: "the unlocked device does not have a reckognizable filesystem on it"?

  - by user824924

  I'm having problems mounting ext4 partitions(inside a LUKS partition) in external drives. The drives are fine, there is no problem whatsoever with the drives and no filesystem corruption. This happened since a recent automatic system upgrade, and a manual upgrade to kernel 3.12.0. It goes like this: I plug in the external drive Passphrase is asked for luks device luks partition correctly unlocked/opened Instead of proceding with mounting the now exposed ext4 partition there's a pop-up saying: "the unlocked device does not have a recognizable filesystem on it". Same happens in this case: $ gvfs-mount -d /dev/sdc2 Enter a passphrase to unlock the volume The passphrase is needed to access encrypted data on WDC WD250... (250 GB Hard Disk). Password: Error mounting /dev/sdc2: The unlocked device does not have a recognizable file system on it Doing a manual sudo mount /dev/dm-1 /mnt/testfolder works with no errors and there is no problem with the filesystem (fscked). Also there doesn’t seem to be anything useful written to dmesg when this happens. What gives?

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• Consistency of DirectX models

  - by marc wellman

  Is there a way to check the consistency of a DirectX model (.x) ? Whilst compiling .x files with XNA GameStudio 3.1 compilation is aborted with the following error message: Error 2 Could not read the X file. The file is corrupt or invalid. Error code: D3DXFERR_PARSEERROR. C:\WFP\Browser\Content\m.x KiviBrowser Some models compile correctly without any error/warning and some abort as described. The files of each model have several thousand lines. I am creating the files in Googles SketchUp 8 where they all look fine and don't show any sign of corruption. Suppose I have such a model my XNA compiler won't compile because their is an inconsistency somewhere in the file - how could I identify this in order to correct it ?

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• Script / App to unRAR files, and only delete the archives which were sucessfully expanded.

  - by Jeremy

  I have a cron job which runs a script to unrar all files in a certain directory (/rared for argument's sake) and place the expanded files in /unrared. I would like to change this script so that it deletes the original rar archives from /rared only if they successfully extracted. This does not mean that unrar has reported that they have been fully extracted, because I have had data corruption during decompression before. Ideally (pie-in-the-sky, just to give you an idea of what I'm shooting for,) the unrar program would include this functionality, comparing an expected md5sum value with the actual md5sum value and only deleting the archive if they match. I don't mind scripting this entire process if I have to, but there must be a better way than unraring twice and comparing md5sums.

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• wubi boots into grub after 12.04 install

  - by josh

  I would just like to recover the files off of the wubi install and then install a proper version of ubuntu, is there any way to recover the files? for further information. I had ubuntu 11.04 and used the update manager to update it to 12.04. it worked for aproximitely a day and then I had to do a hard reset the next day after what seemed to be artifact corruption. upon trying to reboot into ubuntu it brings up the grub command prompt "gnu grub version 1.99-12ubuntu5" i have previously done hard resets on it in the past, there is nothing in my windows hidden 000 recovery folder.

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• Where should people give feedback on Ubuntu?

  - by poolie

  Many people have opinions or want to give feedback about Ubuntu. If they are simple bugs with a crash or display corruption, that's fine, they can file in Launchpad. But, comments about the design or concept of the system are more problematic. They come up all over the place, such as "Some Unity feedback" in other places that are not really about Unity. Ideally the answer will be a place where the comments will actually be read by the design team will tend to encourage people to read the existing comments and not post the same thing again and again perhaps will tend to bubble up good solutions rather than strange ideas will not be annoying to people who are trying to use that place for something else

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• SpriteBatch.Begin() making my model not render correctly

  - by manning18

  I was trying to output some debug information using DrawString when I noticed my model suddenly was being rendered like it was inside-out (like the culling had been disabled or something) and the texture maps weren't applied I commented out the DrawString method until I only had SpriteBatch.Begin() and .End() and that was enough to cause the model rendering corruption - when I commented those calls out the model rendered correctly What could this be a symptom of? I've stripped it down to the barest of code to isolate the problem and this is what I noticed. Draw code below (as stripped down as possible) GraphicsDevice.Clear(Color.LightGray); foreach (ModelMesh mesh in TIEAdvanced.Meshes) { foreach (Effect effect in mesh.Effects) { if (effect is BasicEffect) ((BasicEffect)effect).EnableDefaultLighting(); effect.CurrentTechnique.Passes[0].Apply(); } } spriteBatch.Begin(); spriteBatch.DrawString(spriteFont, "Camera Position: " + cameraPosition.ToString(), new Vector2(10, 10), Color.Blue); spriteBatch.End(); GraphicsDevice.DepthStencilState = DepthStencilState.Default; TIEAdvanced.Draw(Matrix.CreateScale(0.025f), viewMatrix, projectionMatrix);

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• SQLServerCentral Webinar Series #21 - Forgotten Rings & Other Monitoring Stories

  Most common monitoring metrics are important and useful, especially over time, but they can fall short. How do you gather information to determine, for example, if you have buffer cache pressure? Register now for the free webinar. Wednesday, October 17 2012 4:00pm - 5:00pm BST Are you sure you can restore your backups? Run full restore + DBCC CHECKDB quickly and easily with SQL Backup Pro's new automated verification. Check for corruption and prepare for when disaster strikes. Try it now.

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• returning null vs returning zero, which would be better?

  - by Dark Star1

  I inherited a project that I am managing and having to maintain pending the redevelopment of the code base. At the moment I am being tasked with adding little feature all over the place and have gotten into the habit of returning null instead of zero in parts of the code where I am working on. The problem is we have a client that is using this code and parts of code that require data from my implemented features recieve a null and dump the stack trace in UI. I would like to avoid this entirely from my input but without the nullPointer exceptions there's the potential that errors would be introduced into the client's data which may go un-noticed. Usually I would have come up with my own error notification system but I have never inherited a project before. so I am unsure whether to continue down this path. I still believe that the stack dump is preferable to un-noticed data corruption/inaccuracies.

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• SQL in the City - Seattle 2012

  Start the week in Seattle off with a free day of training on Nov 5, 2012 with SQL in the City. Grant Fritchey, Steve Jones and more will be talking SQL Server in the Pacific Northwest. Join us and debate and discuss SQL Server the Red Gate Way. Are you sure you can restore your backups? Run full restore + DBCC CHECKDB quickly and easily with SQL Backup Pro's new automated verification. Check for corruption and prepare for when disaster strikes. Try it now.

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• Is it better to use preprocessor directive or if(constant) statement?

  - by MByD

  Let's say we have a codebase that is used for many different costumer, and we have some code in it that relevant only for costumers of type X. Is it better to use preprocessor directives to include this code only in costumer of type X, or to use if statement, to be more clear: // some code #if TYPE_X_COSTUMER = 1 // do some things #endif // rest of the code or if(TYPE_X_COSTUMER) { // do some things } The arguments I can think about are: Preprocessor directive results in smaller code footprint and less branches (on non-optimizing compilers) If statements results with code that always compiles, e.g. if someone will make a mistake that will harm the irrelevant code for the project he works on, the error will still appear, and he will not corrupt the code base. Otherwise he will not be aware of the corruption. I was always been told to prefer the usage of the processor over the usage of the preprocessor (If this is an argument at all...) What is preferable - when talking about a code base for many different costumers?

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• Oracle E-Business Financials Recommended Patch Collections (RPCs) for R12.1.3 Have Been Released for August 2012

  - by Oracle_EBS

  What is a Recommended Patch Collection (RPC)? An RPC is a collection of recommended patches consolidated into a single, downloadable patch, ready to be applied. The RPCs are created with the following goals in mind: Stability: Address issues that occur often and interfere with the normal completion of crucial business processes, such as period close--as observed by Oracle Development and Global Customer Support. Root Cause Fixes: Deliver a root cause fix for data corruption issues that delay period close, normal transaction flow actions, performance, and other issues. Compact: While bundling a large number of important corrections, we have kept the file footprint as small as possible to facilitate uptake and minimize testing. Reliable: Reliable code with multiple customer downloads and comprehensive testing by QA, Support and Proactive Support. RPCs are available for the following products: Cash Management Collections E-Business Tax Financials for India Fixed Assets General Ledger Internet Expenses iReceivables Loans Payables Payments Receivables Subledger Accounting For the latest Financials Recommended Patch Collections (RPCs), please view: EBS: R12.1 Oracle Financials Recommended Patches [Doc ID 954704.1].

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• What should be the correct ownership of /tmp /var /usr and subdirectories?

  - by sbergeron

  For some reason I think the permissions of those were messed up from a small amount of disk corruption I had. This may be a dupe, I don't know but I googled it and didn't find any mention of it. I need to know how the permissions of them should be set up as currently my computer can't actually use the tmp directory and I can't download anything consequently. I have access to the recovery mode command line but while in sudo user prompts it doesn't seem to work correctly. Also something is wrong with package management and I get dpkg: error: unable to access dpkg status area: read-only file system. I know my system is really messed up but I don't want to have to reinstall AGAIN.

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• GI ????

  - by Allen Gao

  Normal 0 7.8 ? 0 2 false false false MicrosoftInternetExplorer4 classid="clsid:38481807-CA0E-42D2-BF39-B33AF135CC4D" id=ieooui st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:????; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} ??????????11gR2 GI ?????????,??????GI????????????????????? ????????GI???????3???,ohasd??,??????,??????? ??,ohasd??? 1. /etc/inittab?????? h1:35:respawn:/etc/init.d/init.ohasd run >/dev/null 2>&1 </dev/null ???,??????? root 4865 1 0 Dec02 ? 00:01:01 /bin/sh /etc/init.d/init.ohasd run ??????????????,???? +init.ohasd ????????? + os????????? + ??S* ohasd????, ??S96ohasd + GI????????(crsctl enable crs) ??,ohasd.bin ??????,????OLR????,??,??ohasd.bin??????,?????OLR??????????????OLR???$GRID_HOME/cdata/${HOSTNAME}.olr 2. ohasd.bin????????agents(orarootagent, oraagent, cssdagnet ? cssdmonitor) ???????????????,?????agent??????,??????????$GRID_HOME/bin ???????????,??,?????????,??corruption. ???,??????? 1. Mdnsd ??????(Multicast)???????????????????,??????????????????????????? 2. Gpnpd ????,??????????bootstrap ??,??????????????gpnp profile???,?????mdnsd??????,???????????,?????????????,??gpnp profile (<gi_home>/gpnp/profiles/peer/profile.xml)?????????? 3. Gipcd ????,????????????????(cluster interconnect)?????,???????gpnpd???,??,??????????,?????gpnpd ??????? 4. Ocssd.bin ?????????????gpnp profile?????????(Voting Disk),????gpnpd ??????????,?????????????,??ocssd.bin ??????,?????????? + gpnp profile ?????????? + gpnpd ??????? + ??????asm disk ??????????? + ??????????? 5. ??????????:ora.ctssd, ora.asm, ora.cluster_interconnect.haip, ora.crf, ora.crsd ?? ??:????????????????ocssd.bin, gpnpd.bin ? gipcd.bin ????,??gpnpd.bin????,ocssd.bin ? gipcd.bin ?????????,?gpnpd.bin????????,ocssd.bin ? gipcd.bin ????????gpnp profile?????????? ??,????????????,?????crsd????????? 1. Crsd?????????????OCR,????OCR????ASM?,???? ASM??????,??OCR???ASM??????????OCR???????,???????????????? 2. Crsd ?????agents(orarootagent, oraagent_<rdbms_owner>, oraagent_<gi_owner> )???agent????,??????????$GRID_HOME/bin ???????????,??,?????????,??corruption. 3. ????????  ora.net1.network : ????,?????????????,scanvip, vip, listener?????????????,??????????,vip, scanvip ?listener ??offline,?????????????? ora.<scan_name>.vip:scan???vip??,?????3?? ora.<node_name>.vip : ?????vip ?? ora.<listener_name>.lsnr: ???????????????,?11gR2??,listener.ora???????,????????? ora.LISTENER_SCAN<n>.lsnr: scan ????? ora.<????>.dg: ASM ????????????????mount???,dismount???? ora.<????>.db: ???????11gR2????????????,??????????rac ????????,??????????,???????“USR_ORA_INST_NAME@SERVERNAME(<node name> )”???????,??????????ASM???,???????????????????,??dependency?????????,??????????????????,???dependancy???????,??????(crsctl modify res ……)? ora.<???>.svc:?????????11gR2 ??,?????????,???10gR2??,???????????,srv ?cs ????? ora.cvu :?????11.2.0.2???,???????cluvfy??,???????????????? ora.ons : ONS??,????????,????? ??,?????GI??????????????????? $GRID_HOME/log/<node_name>/ocssd <== ocssd.bin ?? $GRID_HOME/log/<node_name>/gpnpd <== gpnpd.bin ?? $GRID_HOME/log/<node_name>/gipcd <== gipcd.bin ?? $GRID_HOME/log/<node_name>/agent/crsd <== crsd.bin ?? $GRID_HOME/log/<node_name>/agent/ohasd <== ohasd.bin ?? $GRID_HOME/log/<node_name>/mdnsd <== mdnsd.bin ?? $GRID_HOME/log/<node_name>/client <== ????GI ??(ocrdump, crsctl, ocrcheck, gpnptool??)??????????? $GRID_HOME/log/<node_name>/ctssd <== ctssd.bin ?? $GRID_HOME/log/<node_name>/crsd <== crsd.bin ?? $GRID_HOME/log/<node_name>/cvu <== cluvfy ????????? $GRID_HOME/bin/diagcollection.sh <== ????????????????? ??,????????(/var/tmp/.oracle ? /tmp/.oracle),??????????????????ipc???,??,?????????????????????,???GI?????????????????????,??????????GI??????????????

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• Program received signal: “0”. warning: check_safe_call: could not restore current frame

  - by varun

  Hi I am getting this error on my application Program received signal: “0”. warning: check_safe_call: could not restore current frame I had also enabled Zombie but its not showing any info about memory corruption in one particular case and showing above error. Please tell me why i am getting this error and how to resolve this. Thanks

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• GZipStream or DeflateStream class?

  - by Seventh Element

  The MSDN documentation tells me the following: The GZipStream class uses the gzip data format, which includes a cyclic redundancy check value for detecting data corruption. The gzip data format uses the same compression algorithm as the DeflateStream class. It seems GZipStream adds some extra data to the output (relative to DeflateStream). I'm wondering, in what type of a scenario would it be essential to use GZipStream and not DeflateStream?

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• BerkeleyDB vs. Tokyo Cabinet

  - by vsedach

  I'm looking for general experiences from people who have used both, particularly on how the two compare on handling large numbers of records, transaction/concurrency/deadlock handling, and juicy stories about database corruption and backup procedures.

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• Freeing memory twice

  - by benjamin button

  Hi, AFAIK, freeing a NULL pointer will result in nothing. I mean nothing is being done by the compiler/no functionality is performed. Still, I do see some statements where people say that one of the scenarios where memory corruption can occur is "freeing memory twice"? Is this still true?

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• How to correctly highlight cursor line in VIM?

  - by Eye of Hell

  Hello. VIM can be configured to highlight current line via :hi cursorline guibg=green and set cursorline commands. But if I enable tabs display via: :hi specialkey guifg=grey guibg=grey :set listchars="tab" :set list Cursor line highlight will corrupt tabs display: Any hints how i can avoid corruption so may tabs are highlighted with one color and cursor line is highlighted with another color without any ^I displayed at intersection?

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• Difference among STLPort and SGI STL

  - by Yan Cheng CHEOK

  Recently, I was buzzed by the following problem STL std::string class causes crashes and memory corruption on multi-processor machines while using VC6. I plan to use an alternative STL libraries instead of the one provided by VC6. I came across 2 libraries : STLPort and SGI STL I was wondering what is the difference between the 2. Which one I should use? Which one able to guarantee thread safety? Thanks.

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• Linux, GNU GCC, ld, version scripts and the ELF binary format -- How does it work??

  - by themoondothshine

  Hey all, I'm trying to learn more about library versioning in Linux and how to put it all to work. Here's the context: -- I have two versions of a dynamic library which expose the same set of interfaces, say libsome1.so and libsome2.so. -- An application is linked against libsome1.so. -- This application uses libdl.so to dynamically load another module, say libmagic.so. -- Now libmagic.so is linked against libsome2.so. Obviously, without using linker scripts to hide symbols in libmagic.so, at run-time all calls to interfaces in libsome2.so are resolved to libsome1.so. This can be confirmed by checking the value returned by libVersion() against the value of the macro LIB_VERSION. -- So I try next to compile and link libmagic.so with a linker script which hides all symbols except 3 which are defined in libmagic.so and are exported by it. This works... Or at least libVersion() and LIB_VERSION values match (and it reports version 2 not 1). -- However, when some data structures are serialized to disk, I noticed some corruption. In the application's directory if I delete libsome1.so and create a soft link in its place to point to libsome2.so, everything works as expected and the same corruption does not happen. I can't help but think that this may be caused due to some conflict in the run-time linker's resolution of symbols. I've tried many things, like trying to link libsome2.so so that all symbols are alised to symbol@@VER_2 (which I am still confused about because the command nm -CD libsome2.so still lists symbols as symbol and not symbol@@VER_2)... Nothing seems to work!!! Help!!!!!! Edit: I should have mentioned it earlier, but the app in question is Firefox, and libsome1.so is libsqlite3.so shipped with it. I don't quite have the option of recompiling them. Also, using version scripts to hide symbols seems to be the only solution right now. So what really happens when symbols are hidden? Do they become 'local' to the SO? Does rtld have no knowledge of their existence? What happens when an exported function refers to a hidden symbol?

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• In C# ,how do I terminate a thread that has had its call stack corrupted?

  - by Emil D

  I have a thread in my application that is running code that can potentially cause call stack corruption ( my application is a testing tool for dlls ). Assuming that I have a method of detecting if the child thread is misbehaving, how would I terminate it? From what I read, calling Thread.Abort() on the misbehaving thread would be equivalent to raising an exception inside it.I fear that that not be a good idea, provided the call stack of the thread might be corrupted.Any suggestions?

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• freeing a memory twice

  - by benjamin button

  Hi, AFAIAK, freeing a NULL will result in nothing.i mean nothing is being done by the compiler/no functionality is performed. Still i do see some statements where people say that one of the scenario,where a memory corruption can occur is "freeing a memory twice"? Is this still true?

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• Graphics card initialisation problems when booting - requires a "double" boot

  - by DMA57361

  Problem Outline When booting from cold (and my machine is disconnected from main power when off, but leaving it connected doesn't help) the graphics card (single PCI-e card GeForce 460) will not initialise on the first boot, leaving me with the motherboards on-board graphics (which kick in automatically if no PCI-e card is found). However, if I restart the computer - normally I do this by powering it off just after the numlock lights up on the keyboard (ie, just after POST/BIOS and before Windows takes over), wait for the system to whirr down, and power up again - the graphics card will work correctly. Once double-booted in this matter the system seems to work correctly - with no noticeable problems. This is reproducible every time I try to boot - it has been working like this for about a month now. Background Information Sept 2010 - I suffered a hardware malfunction (crashes in Windows and graphics corruption on BIOS screens). By way of spare hardware I determined that replacing the PSU removed the issue, so I replaced the PSU with a brand new one of slightly higher power (460W replaced with 500W). Oct 2010 - The problem resurfaced. I purchased a new graphics card (GeForce 460), which removed the problem. The new graphics card immediately started having the boot initialisation problems mentioned. I presumed there was a motherboard fault all along, but because the system worked once booted, and I was temporarily out of spare money, I left the system alone and continued to use it. Early/Mid Dec 2010 - In the space of 5 days I recieved 3 instances of hard drive corruption (seemlingly fixed by chkdsk and sfc in each case...). Since I was already under the impression the motherboard was faulty, I purchased a new one ASAP, this also required new RAM (as I dropped from 4 slots to 2 and didn't want to drop mem quantity). Past 3-4 weeks - With a brand new PSU, Graphics Card, Motherboard and RAM I'm suffering the problem outlined above. So, what could be causing this and how do I can resolve it? Additional Notes Once double-booted the system seems to work entirely correctly. The graphics card problem has occured on two entirely different motherboards. I do not have the opportunity to test the graphics card in a different computer (I've only the old motherboard, which is dubious, or a really old desktop that still has an AGP port). Under load (ie, modern games for long enough for temperatures to plateau) the system remains stable and performs as expected. The software that came with the new motherboard and SpeenFan both report all voltages and temperatures are within nominal bounds, when idle and when under load. I've looking over the BIOS settings for my motherboard multiple times and can find nothing that helps. This system is configured to run with everything at standard levels - no overclocking. I've tried booting the system with only the mobo and graphics card connected (thinking maybe my new PSU was too weak for the new gfx card, even though it meets the quoted PSU requirements for the card) but the same problem persists (and really if the PSU was weak I'd have problems with the system under load). When the gfx card does not initialise the fan on its cooling unit is running, possibly slower than otherwise - but this measurement is by eye and so unreliable.

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

  - by Boris_yo

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

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• DBCC CHECKDB on VVLDB and latches (Or: My Pain is Your Gain)

  - by Argenis

    Does your CHECKDB hurt, Argenis? There is a classic blog series by Paul Randal [blog|twitter] called “CHECKDB From Every Angle” which is pretty much mandatory reading for anybody who’s even remotely considering going for the MCM certification, or its replacement (the Microsoft Certified Solutions Master: Data Platform – makes my fingers hurt just from typing it). Of particular interest is the post “Consistency Options for a VLDB” – on it, Paul provides solid, timeless advice (I use the word “timeless” because it was written in 2007, and it all applies today!) on how to perform checks on very large databases. Well, here I was trying to figure out how to make CHECKDB run faster on a restored copy of one of our databases, which happens to exceed 7TB in size. The whole thing was taking several days on multiple systems, regardless of the storage used – SAS, SATA or even SSD…and I actually didn’t pay much attention to how long it was taking, or even bothered to look at the reasons why - as long as it was finishing okay and found no consistency errors. Yes – I know. That was a huge mistake, as corruption found in a database several days after taking place could only allow for further spread of the corruption – and potentially large data loss. In the last two weeks I increased my attention towards this problem, as we noticed that CHECKDB was taking EVEN LONGER on brand new all-flash storage in the SAN! I couldn’t really explain it, and were almost ready to blame the storage vendor. The vendor told us that they could initially see the server driving decent I/O – around 450Mb/sec, and then it would settle at a very slow rate of 10Mb/sec or so. “Hum”, I thought – “CHECKDB is just not pushing the I/O subsystem hard enough”. Perfmon confirmed the vendor’s observations. Dreaded @BlobEater What was CHECKDB doing all the time while doing so little I/O? Eating Blobs. It turns out that CHECKDB was taking an extremely long time on one of our frankentables, which happens to be have 35 billion rows (yup, with a b) and sucks up several terabytes of space in the database. We do have a project ongoing to purge/split/partition this table, so it’s just a matter of time before we deal with it. But the reality today is that CHECKDB is coming to a screeching halt in performance when dealing with this particular table. Checking sys.dm_os_waiting_tasks and sys.dm_os_latch_stats showed that LATCH_EX (DBCC_OBJECT_METADATA) was by far the top wait type. I remembered hearing recently about that wait from another post that Paul Randal made, but that was related to computed-column indexes, and in fact, Paul himself reminded me of his article via twitter. But alas, our pathologic table had no non-clustered indexes on computed columns. I knew that latches are used by the database engine to do internal synchronization – but how could I help speed this up? After all, this is stuff that doesn’t have a lot of knobs to tweak. (There’s a fantastic level 500 talk by Bob Ward from Microsoft CSS [blog|twitter] called “Inside SQL Server Latches” given at PASS 2010 – and you can check it out here. DISCLAIMER: I assume no responsibility for any brain melting that might ensue from watching Bob’s talk!) Failed Hypotheses Earlier on this week I flew down to Palo Alto, CA, to visit our Headquarters – and after having a great time with my Monkey peers, I was relaxing on the plane back to Seattle watching a great talk by SQL Server MVP and fellow MCM Maciej Pilecki [twitter] called “Masterclass: A Day in the Life of a Database Transaction” where he discusses many different topics related to transaction management inside SQL Server. Very good stuff, and when I got home it was a little late – that slow DBCC CHECKDB that I had been dealing with was way in the back of my head. As I was looking at the problem at hand earlier on this week, I thought “How about I set the database to read-only?” I remembered one of the things Maciej had (jokingly) said in his talk: “if you don’t want locking and blocking, set the database to read-only” (or something to that effect, pardon my loose memory). I immediately killed the CHECKDB which had been running painfully for days, and set the database to read-only mode. Then I ran DBCC CHECKDB against it. It started going really fast (even a bit faster than before), and then throttled down again to around 10Mb/sec. All sorts of expletives went through my head at the time. Sure enough, the same latching scenario was present. Oh well. I even spent some time trying to figure out if NUMA was hurting performance. Folks on Twitter made suggestions in this regard (thanks, Lonny! [twitter]) …Eureka? This past Friday I was still scratching my head about the whole thing; I was ready to start profiling with XPERF to see if I could figure out which part of the engine was to blame and then get Microsoft to look at the evidence. After getting a bunch of good news I’ll blog about separately, I sat down for a figurative smack down with CHECKDB before the weekend. And then the light bulb went on. A sparse column. I thought that I couldn’t possibly be experiencing the same scenario that Paul blogged about back in March showing extreme latching with non-clustered indexes on computed columns. Did I even have a non-clustered index on my sparse column? As it turns out, I did. I had one filtered non-clustered index – with the sparse column as the index key (and only column). To prove that this was the problem, I went and setup a test. Yup, that'll do it The repro is very simple for this issue: I tested it on the latest public builds of SQL Server 2008 R2 SP2 (CU6) and SQL Server 2012 SP1 (CU4). First, create a test database and a test table, which only needs to contain a sparse column: CREATE DATABASE SparseColTest; GO USE SparseColTest; GO CREATE TABLE testTable (testCol smalldatetime SPARSE NULL); GO INSERT INTO testTable (testCol) VALUES (NULL); GO 1000000 That’s 1 million rows, and even though you’re inserting NULLs, that’s going to take a while. In my laptop, it took 3 minutes and 31 seconds. Next, we run DBCC CHECKDB against the database: DBCC CHECKDB('SparseColTest') WITH NO_INFOMSGS, ALL_ERRORMSGS; This runs extremely fast, as least on my test rig – 198 milliseconds. Now let’s create a filtered non-clustered index on the sparse column: CREATE NONCLUSTERED INDEX [badBadIndex] ON testTable (testCol) WHERE testCol IS NOT NULL; With the index in place now, let’s run DBCC CHECKDB one more time: DBCC CHECKDB('SparseColTest') WITH NO_INFOMSGS, ALL_ERRORMSGS; In my test system this statement completed in 11433 milliseconds. 11.43 full seconds. Quite the jump from 198 milliseconds. I went ahead and dropped the filtered non-clustered indexes on the restored copy of our production database, and ran CHECKDB against that. We went down from 7+ days to 19 hours and 20 minutes. Cue the “Argenis is not impressed” meme, please, Mr. LaRock. My pain is your gain, folks. Go check to see if you have any of such indexes – they’re likely causing your consistency checks to run very, very slow. Happy CHECKDBing, -Argenis ps: I plan to file a Connect item for this issue – I consider it a pretty serious bug in the engine. After all, filtered indexes were invented BECAUSE of the sparse column feature – and it makes a lot of sense to use them together. Watch this space and my twitter timeline for a link.

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