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  • In OCaml, how can I create an out_channel which writes to a string/buffer instead of a file on disk

    - by Tianyi Cui
    I have a function of type in_channel -> out_channel -> unit which will output something to an out_channel. Now I'd like to get its output as a string. Creating temporary files to write and read it back seems ugly, so how can I do that? Is there any other methods to create out_channel besides Pervasives.open_out family? Actually, this function implemented a repl. What I really need is to test it programmatically, so I'd like to first wrap it to a function of type string -> string. For creating the in_channel, it seems I can use Scanf.Scanning.from_string, but I don't know how to create the out_channel parameter.

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  • STL ostream_iterator writes to screen even though I overwrote it?

    - by adam_0
    In my code, I have the following: ostream_iterator<double> doubleWriter(cout, " ~ "); // ... *doubleWriter = 1.1; doubleWriter++; *doubleWriter = 2.2; *doubleWriter = 3.3; // shouldn't 2.2 be overwritten? doubleWriter++; *doubleWriter = 44.2; cout << endl << endl; I expected it to output this: 1.1 ~ 3.3 ~ 44.2 ~ Instead, the output was this: 1.1 ~ 2.2 ~ 3.3 ~ 44.2 ~ Why does this happen? It would seem to me that I overwrite 2.2 and stick 3.3 in its spot, since I didn't increment. Is incrementation an optional step?

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  • Does replacing chrome User Data with my own - works without leaving any trace behind? Where else chrome writes data outside of User Data folder?

    - by Selin Peck
    Does replacing chrome User Data with my own - works without leaving any trace behind? Where else chrome writes data outside of User Data folder? I used to start office work by removing chrome User Data, replacing it with my own User Data copied from my external drive, saving the original User Data to other folder. Before leaving in the evening, I will take back my own User Data, and bring back the original User Data where it is originally saved. Is this process advisable? Would I be safe this way or if not, where else does chrome save data outside of User Data folder in AppData? Also, how is the process in Mozilla Firefox?

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  • Why MySQL sat for 2 minutes doing nothing?

    - by Alex R
    This was a one-time thing, not reproducible... But I saved the show innodb status output. Can anybody tell what's going on here? The simple insert took almost 3 minutes to complete. | InnoDB | | ===================================== 110201 15:58:10 INNODB MONITOR OUTPUT ===================================== Per second averages calculated from the last 34 seconds ---------- SEMAPHORES ---------- OS WAIT ARRAY INFO: reservation count 11963, signal count 11766 --Thread 1824 has waited at .\btr\btr0cur.c line 443 for 118.00 seconds the sema phore: S-lock on RW-latch at 09D6453C created in file .\buf\buf0buf.c line 550 a writer (thread id 1824) has reserved it in mode wait exclusive number of readers 1, waiters flag 1 Last time read locked in file .\buf\buf0flu.c line 599 Last time write locked in file .\btr\btr0cur.c line 443 Mutex spin waits 0, rounds 527817, OS waits 7133 RW-shared spins 2532, OS waits 1226; RW-excl spins 1652, OS waits 1118 ------------ TRANSACTIONS ------------ Trx id counter 0 95830 Purge done for trx's n:o < 0 95814 undo n:o < 0 0 History list length 11 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 0 0, not started, OS thread id 3704 MySQL thread id 551, query id 2702112 localhost 127.0.0.1 root show innodb status ---TRANSACTION 0 95829, not started, OS thread id 3132 MySQL thread id 534, query id 2702020 localhost 127.0.0.1 root ---TRANSACTION 0 95828, not started, OS thread id 3152 MySQL thread id 527, query id 2701973 localhost 127.0.0.1 root ---TRANSACTION 0 95827, ACTIVE 118 sec, OS thread id 1824 inserting, thread decl ared inside InnoDB 500 mysql tables in use 1, locked 1 1 lock struct(s), heap size 320, 0 row lock(s) MySQL thread id 526, query id 2701972 localhost 127.0.0.1 root update INSERT INTO log_searchcriteria (userid,search_criteria,date,search_type) VALUES ( NAME_CONST('userid',NULL), NAME_CONST('search_criteria',_latin1' SELECT SQL_C ALC_FOUND_ROWS idx_search.CTCX_LATITUDE, idx_search.CTCX_LONGITUDE, idx_search.b uilding_id, idx_search.LN_LIST_NUMBER, idx_search.LP_LIST_PRICE, idx_search.HSN_ ADRESS_HOUSE_NUMBER, idx_search.STR_ADDRESS_STREET, idx_search.CP_ADDRESS_COMPAS S_POINT, idx_search.UN_UNIT, idx_search.CIT_CITY, idx_search.ZP_ZIP_CODE, idx_se arch.AR_AREA_NAME, idx_search.BR_BEDROOMS, idx_search.BTH_BATHS, idx_search.ST_S TATUS, idx_search.CTCX_STYLE_TYPE, idx_s -------- FILE I/O -------- I/O thread 0 state: wait Windows aio (insert buffer thread) I/O thread 1 state: wait Windows aio (log thread) I/O thread 2 state: wait Windows aio (read thread) I/O thread 3 state: wait Windows aio (write thread) Pending normal aio reads: 0, aio writes: 1, ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0 Pending flushes (fsync) log: 0; buffer pool: 0 151006 OS file reads, 120758 OS file writes, 6844 OS fsyncs 0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s ------------------------------------- INSERT BUFFER AND ADAPTIVE HASH INDEX ------------------------------------- Ibuf: size 1, free list len 5, seg size 7, 24664 inserts, 24664 merged recs, 4612 merges Hash table size 553253, node heap has 629 buffer(s) 0.00 hash searches/s, 0.00 non-hash searches/s --- LOG --- Log sequence number 5 2318193115 Log flushed up to 5 2318193115 Last checkpoint at 5 2318129891 0 pending log writes, 0 pending chkp writes 3036 log i/o's done, 0.00 log i/o's/second ---------------------- BUFFER POOL AND MEMORY ---------------------- Total memory allocated 213459462; in additional pool allocated 1720192 Dictionary memory allocated 240416 Buffer pool size 8192 Free buffers 0 Database pages 7563 Modified db pages 18 Pending reads 0 Pending writes: LRU 0, flush list 18, single page 0 Pages read 150973, created 28788, written 115137 0.00 reads/s, 0.00 creates/s, 0.00 writes/s No buffer pool page gets since the last printout -------------- ROW OPERATIONS -------------- 1 queries inside InnoDB, 0 queries in queue 1 read views open inside InnoDB Main thread id 2992, state: flushing buffer pool pages Number of rows inserted 794294, updated 89203, deleted 13698, read 1453084305 0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s ---------------------------- END OF INNODB MONITOR OUTPUT ============================ Thanks

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  • flashcache with mdadm and LVM

    - by Backtogeek
    I am having trouble setting up flashcache on a system with LVM and mdadm, I suspect I am either just missing an obvious step or getting some mapping wrong and hoped someone could point me in the right direction? system info: CentOS 6.4 64 bit mdadm config md0 : active raid1 sdd3[2] sde3[3] sdf3[4] sdg3[5] sdh3[1] sda3[0] 204736 blocks super 1.0 [6/6] [UUUUUU] md2 : active raid6 sdd5[2] sde5[3] sdf5[4] sdg5[5] sdh5[1] sda5[0] 3794905088 blocks super 1.1 level 6, 512k chunk, algorithm 2 [6/6] [UUUUUU] md3 : active raid0 sdc1[1] sdb1[0] 250065920 blocks super 1.1 512k chunks md1 : active raid10 sdh1[1] sda1[0] sdd1[2] sdf1[4] sdg1[5] sde1[3] 76749312 blocks super 1.1 512K chunks 2 near-copies [6/6] [UUUUUU] pcsvan PV /dev/mapper/ssdcache VG Xenvol lvm2 [3.53 TiB / 3.53 TiB free] Total: 1 [3.53 TiB] / in use: 1 [3.53 TiB] / in no VG: 0 [0 ] flashcache create command used: flashcache_create -p back ssdcache /dev/md3 /dev/md2 pvdisplay --- Physical volume --- PV Name /dev/mapper/ssdcache VG Name Xenvol PV Size 3.53 TiB / not usable 106.00 MiB Allocatable yes PE Size 128.00 MiB Total PE 28952 Free PE 28912 Allocated PE 40 PV UUID w0ENVR-EjvO-gAZ8-TQA1-5wYu-ISOk-pJv7LV vgdisplay --- Volume group --- VG Name Xenvol System ID Format lvm2 Metadata Areas 1 Metadata Sequence No 2 VG Access read/write VG Status resizable MAX LV 0 Cur LV 1 Open LV 1 Max PV 0 Cur PV 1 Act PV 1 VG Size 3.53 TiB PE Size 128.00 MiB Total PE 28952 Alloc PE / Size 40 / 5.00 GiB Free PE / Size 28912 / 3.53 TiB VG UUID 7vfKWh-ENPb-P8dV-jVlb-kP0o-1dDd-N8zzYj So that is where I am at, I thought that was the job done however when creating a logical volume called test and mounting it is /mnt/test the sequential write is pathetic, 60 ish MB/s /dev/md3 has 2 x SSD's in Raid0 which alone is performing at around 800 MB/s sequential write and I am trying to cache /dev/md2 which is 6 x 1TB drives in raid6 I have read a number of pages through the day and some of them here, it is obvious from the results that the cache is not functioning but I am unsure why. I have added the filter line in the lvm.conf filter = [ "r|/dev/sdb|", "r|/dev/sdc|", "r|/dev/md3|" ] It is probably something silly but the cache is clearly performing no writes so I suspect I am not mapping it or have not mounted the cache correctly. dmsetup status ssdcache: 0 7589810176 flashcache stats: reads(142), writes(0) read hits(133), read hit percent(93) write hits(0) write hit percent(0) dirty write hits(0) dirty write hit percent(0) replacement(0), write replacement(0) write invalidates(0), read invalidates(0) pending enqueues(0), pending inval(0) metadata dirties(0), metadata cleans(0) metadata batch(0) metadata ssd writes(0) cleanings(0) fallow cleanings(0) no room(0) front merge(0) back merge(0) force_clean_block(0) disk reads(9), disk writes(0) ssd reads(133) ssd writes(9) uncached reads(0), uncached writes(0), uncached IO requeue(0) disk read errors(0), disk write errors(0) ssd read errors(0) ssd write errors(0) uncached sequential reads(0), uncached sequential writes(0) pid_adds(0), pid_dels(0), pid_drops(0) pid_expiry(0) lru hot blocks(31136000), lru warm blocks(31136000) lru promotions(0), lru demotions(0) Xenvol-test: 0 10485760 linear I have included as much info as I can think of, look forward to any replies.

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  • Innodb Queries Slow

    - by user105196
    I have redHat 5.3 (Tikanga) with Mysql 5.0.86 configued with RIAD 10 HW, I run an application inquiries from Mysql/InnoDB and MyIsam tables, the queries are super fast,but some quires on Innodb tables sometime slow down and took more than 1-3 seconds to run and these queries are simple and optimized, this problem occurred just on innodb tables in different time with random queries. Why is this happening only to Innodb tables? the below is the Innodb status and some Mysql variables: show innodb status\G ************* 1. row ************* Status: 120325 10:54:08 INNODB MONITOR OUTPUT Per second averages calculated from the last 19 seconds SEMAPHORES OS WAIT ARRAY INFO: reservation count 22943, signal count 22947 Mutex spin waits 0, rounds 561745, OS waits 7664 RW-shared spins 24427, OS waits 12201; RW-excl spins 1461, OS waits 1277 TRANSACTIONS Trx id counter 0 119069326 Purge done for trx's n:o < 0 119069326 undo n:o < 0 0 History list length 41 Total number of lock structs in row lock hash table 0 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 0 0, not started, process no 29093, OS thread id 1166043456 MySQL thread id 703985, query id 5807220 localhost root show innodb status FILE I/O I/O thread 0 state: waiting for i/o request (insert buffer thread) I/O thread 1 state: waiting for i/o request (log thread) I/O thread 2 state: waiting for i/o request (read thread) I/O thread 3 state: waiting for i/o request (write thread) Pending normal aio reads: 0, aio writes: 0, ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0 Pending flushes (fsync) log: 0; buffer pool: 0 132777 OS file reads, 689086 OS file writes, 252010 OS fsyncs 0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s INSERT BUFFER AND ADAPTIVE HASH INDEX Ibuf: size 1, free list len 366, seg size 368, 62237 inserts, 62237 merged recs, 52881 merges Hash table size 8850487, used cells 3698960, node heap has 7061 buffer(s) 0.00 hash searches/s, 0.00 non-hash searches/s LOG Log sequence number 15 3415398745 Log flushed up to 15 3415398745 Last checkpoint at 15 3415398745 0 pending log writes, 0 pending chkp writes 218214 log i/o's done, 0.00 log i/o's/second BUFFER POOL AND MEMORY Total memory allocated 4798817080; in additional pool allocated 12342784 Buffer pool size 262144 Free buffers 101603 Database pages 153480 Modified db pages 0 Pending reads 0 Pending writes: LRU 0, flush list 0, single page 0 Pages read 151954, created 1526, written 494505 0.00 reads/s, 0.00 creates/s, 0.00 writes/s No buffer pool page gets since the last printout ROW OPERATIONS 0 queries inside InnoDB, 0 queries in queue 1 read views open inside InnoDB Main thread process no. 29093, id 1162049856, state: waiting for server activity Number of rows inserted 77675, updated 85439, deleted 0, read 14377072495 0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s END OF INNODB MONITOR OUTPUT 1 row in set, 1 warning (0.02 sec) read_buffer_size = 128M sort_buffer_size = 256M tmp_table_size = 1024M innodb_additional_mem_pool_size = 20M innodb_log_file_size=10M innodb_lock_wait_timeout=100 innodb_buffer_pool_size=4G join_buffer_size = 128M key_buffer_size = 1G can any one help me ?

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  • SBS 2008 SP2 Backup - Volume Shadow Copy Operation Failed

    - by Robert Ortisi
    Server Setup Exchange 2007 Version: 08.03.0192.001 (Rollup 4) Windows Small Business Server 2008 SP2 (Rollup 5) Exchange set up on D: drive (449 GB / 698 GB Free) 80 GB / 148 GB Free on OS drive. Issue Backup Failure (VSS related) Backup Software Windows Server Backup (ver 1.0) Simplified Error Creation of the shared protection point timed out. Unknown error (0x81000101) The flush and hold writes operation on volume C: timed out while waiting for a release writes command. Volume Shadow Copy Warning: VSS spent 43 seconds trying to flush and hold the volume \?\Volume{b562a5dd-8246-11de-a75b-806e6f6e6963}. This might cause problems when other volumes in the shadow-copy set timeout waiting for the release-writes phase, and it can cause the shadow-copy creation to fail. Trying again when disk activity is lower may solve this problem. What I've tried Server Reboot. Updated Server and Exchange. ReConfigured Sharepoint (Helped resolve last vss error I encountered). registered VSS Dll's (Backups will sometimes work afterwards but VSS writers fail soon after). Tried Implementing Hotfix: http://support.microsoft.com/kb/956136 Tried Implementing Hotfix: http://support.microsoft.com/kb/972135 I left it for a few days and a few backups came through but then began to fail again. Detailed Information Log Name: Application Source: VSS Date: 16/11/2011 8:02:11 PM Event ID: 12341 Task Category: None Level: Warning Keywords: Classic User: N/A Computer: SERVER.DOMAIN.local Description: Volume Shadow Copy Warning: VSS spent 43 seconds trying to flush and hold the volume \?\Volume{b562a5dd-8246-11de-a75b-806e6f6e6963}. This might cause problems when other volumes in the shadow-copy set timeout waiting for the release-writes phase, and it can cause the shadow-copy creation to fail. Trying again when disk activity is lower may solve this problem. Operation: Executing Asynchronous Operation Context: Current State: flush-and-hold writes Volume Name: \?\Volume{b562a5dd-8246-11de-a75b-806e6f6e6963}\ Event Xml: 12341 3 0 0x80000000000000 1651049 Application SERVER.DOMAIN.local 43 \?\Volume{b562a5dd-8246-11de-a75b-806e6f6e6963}\ Operation: Executing Asynchronous Operation Context: Current State: flush-and-hold writes Volume Name: \?\Volume{b562a5dd-8246-11de-a75b-806e6f6e6963}\ ================================================================================= Log Name: System Source: volsnap Date: 16/11/2011 8:02:11 PM Event ID: 8 Task Category: None Level: Error Keywords: Classic User: N/A Computer: SERVER.DOMAIN.local Description: The flush and hold writes operation on volume C: timed out while waiting for a release writes command. Event Xml: 8 2 0 0x80000000000000 987135 System SERVER.DOMAIN.local ================================================================================== Log Name: Application Source: Microsoft-Windows-Backup Date: 16/11/2011 8:11:18 PM Event ID: 521 Task Category: None Level: Error Keywords: User: SYSTEM Computer: SERVER.DOMAIN.local Description: Backup started at '16/11/2011 9:00:35 AM' failed as Volume Shadow copy operation failed for backup volumes with following error code '2155348001'. Please rerun backup once issue is resolved. Event Xml: 521 0 2 0 0 0x8000000000000000 1651065 Application SERVER.DOMAIN.local 2011-11-16T09:00:35.446Z 2155348001 %%2155348001 ================================================================================== Writer name: 'FRS Writer' Writer Id: {d76f5a28-3092-4589-ba48-2958fb88ce29} Writer Instance Id: {ba047fc6-9ce8-44ba-b59f-f2f8c07708aa} State: [5] Waiting for completion Last error: No error Writer name: 'ASR Writer' Writer Id: {be000cbe-11fe-4426-9c58-531aa6355fc4} Writer Instance Id: {0aace3e2-c840-4572-bf49-7fcc3fbcf56d} State: [1] Stable Last error: No error Writer name: 'Shadow Copy Optimization Writer' Writer Id: {4dc3bdd4-ab48-4d07-adb0-3bee2926fd7f} Writer Instance Id: {054593e2-2086-4480-92e5-30386509ed1b} State: [1] Stable Last error: No error Writer name: 'Registry Writer' Writer Id: {afbab4a2-367d-4d15-a586-71dbb18f8485} Writer Instance Id: {840e6f5f-f35a-4b65-bb20-060cf2ee892a} State: [1] Stable Last error: No error Writer name: 'COM+ REGDB Writer' Writer Id: {542da469-d3e1-473c-9f4f-7847f01fc64f} Writer Instance Id: {9486bedc-f6e8-424b-b563-8b849d51b1e1} State: [1] Stable Last error: No error Writer name: 'BITS Writer' Writer Id: {4969d978-be47-48b0-b100-f328f07ac1e0} Writer Instance Id: {29368bb3-e04b-4404-8fc9-e62dae18da91} State: [1] Stable Last error: No error Writer name: 'Dhcp Jet Writer' Writer Id: {be9ac81e-3619-421f-920f-4c6fea9e93ad} Writer Instance Id: {cfb58c78-9609-4133-8fc8-f66b0d25e12d} State: [5] Waiting for completion Last error: No error ==================================================================================

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  • How to write a cgi script in perl that accepts output (an image file) from a urlconnection in a Java applet and writes it to the server?

    - by Brad Rock
    I created a Java applet for my web page that creates an image file. I want users to be able to run the applet and create unique images, click a button, and have the image they created be saved to the web server. I think I have the code down for writing the image to a URLconnection in the Java applet itself after having successfully written a file while running the applet on my system rather than the web page and saving a file to the local disk, and then altering a few things to write to a URLConnection instead of to the local disk (although we shall see how that works too). I am now trying to write a cgi script in perl that takes the image output from the URLConnection and writes the image to a file on my web server (note: I am a newbie to perl). I have found many examples of how to do something similar with simple text coming from an applet and then writing it to a text file, but I want to know how to apply the same concept to an image. Particularly, how do I read in the image? I've seen text input get read by using read(STDIN, $some_variable)--does the same thing work with an image? Likewise, how do I write the image file? What function do I use? Thanks for your help. I know that I am rather naive about all of this.

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  • Why does x86 WiX installer on Vista x64 not write keys to WOW6432Node in the registry

    - by Ryan Conrad
    I have an installer that writes to HKLM\Software\DroidExplorer\InstallPath. On any x86 machine it writes just fine to the expected location, on Windows XP x64 and Windows 7 x64 it also writes to the expected location, which is actually HKLM\Software\WOW6432Node\DroidExplorer\InstallPath. Later on during the install, my bootstrapper, which is also x86, attempts to read the value. On all x86 Windows machines it is successful, and on Windows XP x64 and Windows 7 x64, but Windows Vista x64 is unable to locate the key. If I look in the registry, it doesn't actually write it to WOW6432Node on Vista, it writes it to Software\DroidExplorer\InstallPath If I do not forcefully tell the installer to write to WOW6432Node, it writes the value to Software\DroidExplorer\InstallPath, but the bootstrapper still trys to look in WOW6432Node because of the Registry Reflection. This is on all x64 systems. Why is Vista x64 the only one I have this issue with? Is there a way around this?

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  • Digital Storage for Airline Entertainment

    - by Bill Evjen
    by Thomas Coughlin Common flash memory cards The most common flash memory products currently in use are SD cards and derivative products (e.g. mini and micro-SD cards) Some compact flash used for professional applications (such as DSLR cameras) Evolution of leading flash formats Standardization –> market expansion Market expansion –> volume iNAND –> focus is on enabling embedded X3 iSSD –> ideal for thin form factor devices Flash memory applications Phones are the #1 user of flash memory Flash memory is used as embedded and removable storage in many mobile applications Flash memory is being used in computers as USB sticks and SSDs Possible use of flash memory in computer combined with HDDs (hybrid HDDs and paired or dual storage computers) It can be a removable card or an embedded card These devices can only handle a specific number of writes Flash memory reads considerably quicker than hard drives Hybrid and dual storage in computers SSDs can provide fast performance but they are expensive HDDs can provide cheap storage but they are relatively slow Combining some flash memory with a HDD can provide costs close to those of HDDs and performance close to flash memory Seagate Momentus XT hybrid HDD Various dual storage offerings putting flash memory with HDDs Other common flash memory devices USB sticks All forms and colors Used for moving files around Some sold with content on them (Sony Movies on USB sticks) Solid State Drives (SSDs) Floating Gate Flash Memory Cell When a bit is programmed, electrons are stored upon the floating gate This has the effect of offsetting the charge on the control gate of the transistor If there is no charge upon the floating gate, then the control gate’s charge determines whether or not a current flows through the channel A strong charge on the control gate assumes that no current flows. A weak charge will allow a strong current to flow through. Similar to HDDs, flash memory must provide: Bit error correction Bad block management NAND and NOR memories are treated differently when it comes to managing wear In many NOR-based systems no management is used at all, since the NOR is simply used to store code, and data is stored in other devices. In this case, it would take a near-infinite amount of time for wear to become an issue since the only time the chip would see an erase/write cycle is when the code in the system is being upgraded, which rarely if ever happens over the life of a typical system. NAND is usually found in very different application than is NOR Flash memory wears out This is expected to get worse over time Retention: Disappearing data Bits fade away Retention decreases with increasing read/writes Bits may change when adjacent bits are read Time and traffic are concerns Controllers typically groom read disturb errors Like DRAM refresh Increases erase/write frequency Application characteristics Music – reads high / writes very low Video – r high / writes very low Internet Cache – r high / writes low On airplanes Many consumers now have their own content viewing devices – do they need the airlines? Is there a way to offer more to consumers, especially with their own viewers Additional special content tie into airplane network access to electrical power, internet Should there be fixed embedded or removable storage for on-board airline entertainment? Is there a way to leverage personal and airline viewers and content in new and entertaining ways?

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  • NFSv3 Asynchronous Write Depends on Block Size?

    - by Joe Swanson
    I am trying to figure out if my NFSv3 deployment is performing SAFE asynchronous writes. I suspect that it is doing strictly synchronous writes, as I am getting poor performance in general. I used Wireshark to look at the 'stable' flag in write calls, and look for 'commit' calls. I noticed that, with especially large block sizes, writes to appear to be performed asynchronously: dd if=/dev/zero of=/proj/re3/0/zero bs=2097152 count=512 However, smaller block sizes appear to be performed strictly synchronously: dd if=/dev/zero of=/proj/re3/0/zero bs=8192 count=655360 What gives? How does the client decide whether to tell the server to perform writes synchronously or asynchronously? Is there any way I can get smaller block sizes to be performed asynchronously?

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  • SSD with multiple partitions - disk life implications

    - by Nicolas Webb
    Each block on a SSD has a finite number of writes. This is mitigated on modern drives by "spreading" the writes around as you use the drive. I'm wondering if you partition a SSD into several partitions (a Mac using Boot Camp, for example) if this measure is defeated somewhat - can the writes be spread across the entire drive? Or are they contained strictly within the partition boundaries? Any SSD controller engineers here :)?

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  • Why does dstat show zeroes for disk activity on my virtual private server running Ubuntu?

    - by Jonathan Berger
    I'm trying to monitor the number of disk reads and writes on my VPS (Rackspace in this case) running Ubuntu 9.04. I realize there are many tools to do this, but when using dstat 0.7 I tried the following command: dstat -d The output is just two columns of zeroes even when I upload a large file via scp that should be causing a large number of disk writes. Why is this, and how do I get dstat to correctly display the number of disk reads and writes?

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  • Can enabling a RAID controller's writeback cache harm overall performance?

    - by Nathan O'Sullivan
    I have an 8 drive RAID 10 setup connected to an Adaptec 5805Z, running Centos 5.5 and deadline scheduler. A basic dd read test shows 400mb/sec, and a basic dd write test shows about the same. When I run the two simultaneously, I see the read speed drop to ~5mb/sec while the write speed stays at more or less the same 400mb/sec. The output of iostat -x as you would expect, shows that very few read transactions are being executed while the disk is bombarded with writes. If i turn the controller's writeback cache off, I dont see a 50:50 split but I do see a marked improvement, somewhere around 100mb/s reads and 300mb/s writes. I've also found if I lower the nr_requests setting on the drive's queue (somewhere around 8 seems optimal) I can end up with 150mb/sec reads and 150mb/sec writes; ie. a reduction in total throughput but certainly more suitable for my workload. Is this a real phenomenon? Or is my synthetic test too simplistic? The reason this could happen seems clear enough, when the scheduler switches from reads to writes, it can run heaps of write requests because they all just land in the controllers cache but must be carried out at some point. I would guess the actual disk writes are occuring when the scheduler starts trying to perform reads again, resulting in very few read requests being executed. This seems a reasonable explanation, but it also seems like a massive drawback to using writeback cache on an system with non-trivial write loads. I've been searching for discussions around this all afternoon and found nothing. What am I missing?

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  • Top 10 Linked Blogs of 2010

    - by Bill Graziano
    Each week I send out a SQL Server newsletter and include links to interesting blog posts.  I’ve linked to over 500 blog posts so far in 2010.  Late last year I started storing those links in a database so I could do a little reporting.  I tend to link to posts related to the OLTP engine.  I also try to link to the individual blogger in the group blogs.  Unfortunately that wasn’t possible for the SQLCAT and CSS blogs.  I also have a real weakness for posts related to PASS. These are the top 10 blogs that I linked to during the year ordered by the number of posts I linked to. Paul Randal – Paul writes extensively on the internals of the relational engine.  Lots of great posts around transactions, transaction log, disaster recovery, corruption, indexes and DBCC.  I also linked to many of his SQL Server myths posts. Glenn Berry – Glenn writes very interesting posts on how hardware affects SQL Server.  I especially like his posts on the various CPU platforms.  These aren’t necessarily topics that I’m searching for but I really enjoy reading them. The SQLCAT Team – This Microsoft team focuses on the largest and most interesting SQL Server installations.  The regularly publish white papers and best practices. SQL Server CSS Team – These are the top engineers from the Microsoft Customer Service and Support group.  These are the folks you finally talk to after your case has been escalated about 20 times.  They write about the interesting problems they find. Brent Ozar – The posts I linked to mostly focused on the relational engine: CPU, NUMA, SSD drives, performance monitoring, etc.  But Brent writes about a real variety of topics including blogging, social networking, speaking, the MCM, SQL Azure and anything else that seems to strike his fancy.  His posts are always well written and though provoking. Jeremiah Peschka – A number of Jeremiah’s posts weren’t about SQL Server.  He’s very active in the “NoSQL” area and I linked to a number of those posts.  I think it’s important for people to know what other technologies are out there. Brad McGehee – Brad writes about being a DBA including maintenance plans, DBA checklists, compression and audit. Thomas LaRock – I linked to a variety of posts from PBM to networking to 24 Hours of PASS to TDE.  Just a real variety of topics.  Tom always writes with an interesting style usually mixing in a movie theme and/or bacon. Aaron Bertrand – Many of my links this year were Denali features.  He also had a great series on bad habits to kick. Michael J. Swart – This last one surprised me.  There are some well known SQL Server bloggers below Michael on this list.  I linked to posts on indexes, hierarchies, transactions and I/O performance and a variety of other engine related posts.  All are interesting and well thought out.  Many of his non-SQL posts are also very good.  He seems to have an interest in puzzles and other brain teasers.  Michael, I won’t be surprised again!

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  • Saving a reference to a int.

    - by Scott Chamberlain
    Here is a much simplified version of what I am trying to do static void Main(string[] args) { int test = 0; int test2 = 0; Test A = new Test(ref test); Test B = new Test(ref test); Test C = new Test(ref test2); A.write(); //Writes 1 should write 1 B.write(); //Writes 1 should write 2 C.write(); //Writes 1 should write 1 Console.ReadLine(); } class Test { int _a; public Test(ref int a) { _a = a; //I loose the reference here } public void write() { var b = System.Threading.Interlocked.Increment(ref _a); Console.WriteLine(b); } } In my real code I have a int that will be incremented by many threads however where the threads a called it will not be easy to pass it the parameter that points it at the int(In the real code this is happening inside a IEnumerator). So a requirement is the reference must be made in the constructor. Also not all threads will be pointing at the same single base int so I can not use a global static int either. I know I can just box the int inside a class and pass the class around but I wanted to know if that is the correct way of doing something like this? What I think could be the correct way: static void Main(string[] args) { Holder holder = new Holder(0); Holder holder2 = new Holder(0); Test A = new Test(holder); Test B = new Test(holder); Test C = new Test(holder2); A.write(); //Writes 1 should write 1 B.write(); //Writes 2 should write 2 C.write(); //Writes 1 should write 1 Console.ReadLine(); } class Holder { public Holder(int i) { num = i; } public int num; } class Test { Holder _holder; public Test(Holder holder) { _holder = holder; } public void write() { var b = System.Threading.Interlocked.Increment(ref _holder.num); Console.WriteLine(b); } } Is there a better way than this?

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  • Do you do unit tests for non production code?

    - by Ikaso
    I am interested in the following scenario specifically. Suppose you have team that writes production code and a team that writes automatic tests. The team that writes automatic tests has a dedicated framework intended to write the automatic tests. Should the testing team write unit tests for their framework although the framework is not used in production?

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  • Slower/cached Linux file system required

    - by Chopper3
    I know it sounds odd but I need a slower or cached filesystem. I have a lot of firewalls that are syslog'ing their data to a pair of Linux VMs which write these files to their 'local' (actually FC SAN attached) ext3-formatted disks and also forward the messages to our Splunk servers. The problem is that the syslog server is writing these syslog messages as hundreds, sometimes thousands, of tiny ~4k writes per second back to our FC SAN - which can handle this workload right now but our FW traffic's going to be growing by at least a factor of 5000% (really) in coming months and that'll be a pain for the SAN, I want to fix the root cause before it's a problem. So I need some help figuring out a way of getting these writes cached or held-off in some way from the 'physical' disks so that the VMs fire off larger, but less frequent, writes - there's no way of avoiding these writes but there's no need for it to do so many tiny ones. I've looked at the various ext3 options, setting noatime and nodiratime but that's not made much of a dent in the problem. Obviously I'm investigating other file systems but thought I'd throw this out in case others have the same problem in the future. Oh and I can't just forward these messages to Splunk, our firewall team insist they're in their original format for diag purposes.

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  • limit linux background flush (dirty pages)

    - by korkman
    Background flushing in linux happens when either too much written data is pending (adjustable via /proc/sys/vm/dirty_background_ratio) or a timeout for pending writes is reached (/proc/sys/vm/dirty_expire_centisecs). Unless another limit is being hit (/proc/sys/vm/dirty_ratio), more written data may be cached. Further writes will block. In theory, this should create a background process writing out dirty pages without disturbing other processes. In practice, it does disturb any process doing uncached reading or synchronous writing. Badly. This is because the background flush actually writes at 100% device speed and any other device requests at this time will be delayed (because all queues and write-caches on the road are filled). Is there any way to limit the amount of requests per second the flushing process performs, or otherwise effectively prioritize other device I/O?

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  • write-through RAM disk, or massive caching of file system?

    - by Will
    I have a program that is very heavily hitting the file system, reading and writing randomly to a set of working files. The files total several gigabytes in size, but I can spare the RAM to keep them all mostly in memory. The machines this program runs on are typically Ubuntu Linux boxes. Is there a way to configure the file system to have a very very large cache, and even to cache writes so they hit the disk later? I understand the issues with power loss or such, and am prepared to accept that. Crashing aside, in normal operation the writes should eventually reach the disk! Or is there a way to create a RAM disk that writes-through to real disk?

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  • Execution plan different on two different Sql Servers

    - by Lieven Cardoen
    On our sql server, a query takes 20 ms. If I look at the execution plan, parallelism is used, hash match, Bitmap create, ... a lot of images with two arrows pointing to the left. On sql server of a customer where our product runs, the same query takes 2500 ms. If I look at their execution plan, no parallelism or any of the things with arrows are used... I've been searching for a couple of days no why the query runs so much slower on their sql server. Is parallelism and all of the other things something that can be configured on their sql server? And how to you configure that? What are the dangers of using parallelism? Another strange thing is that on our server, the query needs some 1200 reads and 0 writes. On their sql server it needs 1.5 million reads and some 1500 writes. Why these writes when a read query is done?

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  • Utility to record IO statistics (random/sequential, block sizes, read/write ratio) in Unix

    - by Michael Pearson
    As part of provisioning our new server (see other SF) I'd like to find out the following: ratio of random to sequential reads & writes amount of data read & written at a time (pref in histogram form) I can already figure out our reads/writes on a per-operation and overall data level using iostat & dstat, but I'd like to know more. For example, I'd like to know that we're mostly random 16kb reads, or a lot of sequential 64kb reads with random writes. We're (currently) on an Ubuntu 10.04 VM. Is there a utility that I can run that will record and present this information for me?

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