[PERFORM] High load and iowait but no disk access
We have been trying to pinpoint what originally seem to be a I/O bottleneck but which now seems to be an issue with either Postgresql or RHES 3. We have the following test environment on which we can reproduce the problem: 1) Test System A Dell 6650 Quad Xeon Pentium 4 8 Gig of RAM OS: RHES 3 update 2 Storage: NetApp FAS270 connected using an FC card using 10 disks 2) Test System B Dell Dual Xeon Pentium III 2 Gig o RAM OS: RHES 3 update 2 Storage: NetApp FAS920 connected using an FC card using 28 disks Our Database size is around 30G. The behavior we see is that when running queries that do random reads on disk, IOWAIT goes over 80% and actual disk IO falls to a crawl at a throughput bellow 3000kB/s (We usually average 4 kB/s to 8 kB/s on sequential read operations on the netapps) The stats of the NetApp do confirm that it is sitting idle. Doing an strace on the Postgresql process shows that is it doing seeks and reads. So my question is where is this iowait time spent ? Is there a way to pinpoint the problem in more details ? We are able to reproduce this behavior with Postgresql 7.4.8 and 8.0.3 I have included the output of top,vmstat,strace and systat from the Netapp from System B while running a single query that generates this behavior. Rémy top output: 06:27:28 up 5 days, 16:59, 6 users, load average: 1.04, 1.30, 1.01 72 processes: 71 sleeping, 1 running, 0 zombie, 0 stopped CPU states: cpuusernice systemirq softirq iowaitidle total2.7%0.0%1.0% 0.1% 0.2% 46.0% 49.5% cpu000.2%0.0%0.2% 0.0% 0.2%2.2% 97.2% cpu015.3%0.0%1.9% 0.3% 0.3% 89.8%1.9% Mem: 2061696k av, 2043936k used, 17760k free, 0k shrd,3916k buff 1566332k actv, 296648k in_d, 30504k in_c Swap: 16771584k av, 21552k used, 16750032k free 1933772k cached PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND 30960 postgres 15 0 13424 10M 9908 D 2.7 0.5 2:00 1 postmaster 30538 root 15 0 1080 764 524 S 0.7 0.0 0:43 0 sshd 1 root 15 0 496 456 436 S 0.0 0.0 0:08 0 init 2 root RT 0 00 0 SW0.0 0.0 0:00 0 migration/0 3 root RT 0 00 0 SW0.0 0.0 0:00 1 migration/1 4 root 15 0 00 0 SW0.0 0.0 0:01 0 keventd 5 root 34 19 00 0 SWN 0.0 0.0 0:00 0 ksoftirqd/0 6 root 34 19 00 0 SWN 0.0 0.0 0:00 1 ksoftirqd/1 9 root 15 0 00 0 SW0.0 0.0 0:24 1 bdflush 7 root 15 0 00 0 SW0.0 0.0 6:53 1 kswapd 8 root 15 0 00 0 SW0.0 0.0 8:44 1 kscand 10 root 15 0 00 0 SW0.0 0.0 0:13 0 kupdated 11 root 25 0 00 0 SW0.0 0.0 0:00 0 mdrecoveryd 17 root 15 0 00 0 SW0.0 0.0 0:00 0 ahc_dv_0 vmstat output procs memory swap io system cpu r b swpd free buff cache si sobibo incs us sy id wa 0 1 21552 17796 4872 193192823 3 1 27 6 2 1 7 3 0 1 21552 18044 4880 193165200 1652 0 397 512 1 2 50 47 0 1 21552 17976 4896 193166400 2468 0 407 552 2 2 50 47 1 0 21552 17984 4896 193160800 2124 0 418 538 3 3 48 46 0 1 21552 18028 4900 193153600 1592 0 385 509 1 3 50 46 0 1 21552 18040 4916 193148800 1620 820 419 581 2 2 50 46 0 1 21552 17968 4916 193153604 1708 4 402 554 3 1 50 46 1 1 21552 18052 4916 193138800 1772 0 409 531 3 1 49 47 0 1 21552 17912 4924 193149200 1772 0 408 565 3 1 48 48 0 1 21552 17932 4932 193144004 1356 4 391 545 5 0 49 46 0 1 21552 18320 4944 193101604 1500 840 414 571 1 1 48 50 0 1 21552 17872 4944 193144000 2116 0 392 496 1 5 46 48 0 1 21552 18060 4944 193123200 2232 0 423 597 1 2 48 49 1 1 21552 17684 4944 193158400 1752 0 395 537 1 1 50 48 0 1 21552 18000 4944 193124000 1576 0 401 549 0 1 50 49 NetApp stats: CPU NFS CIFS HTTP TotalNet kB/s Disk kB/s Tape kB/s Cache Cache CP CP Disk DAFS FCP iSCSI FCP kB/s in out read write read write age hit time ty util in out 2% 0 0 0 139 0 0 2788 0 0 0 3 96% 0% - 15% 0 139 0 3 2277 2% 0 0 0 144 0 0 2504 0 0 0 3 96% 0% - 18% 0 144 0 3 2150 2% 0 0 0 130 0 0 2212 0 0 0 3 96% 0% - 13% 0 130 0 3 1879 3% 0 0 0 169 0 0 2937 80 0
Re: [PERFORM] High load and iowait but no disk access
On 30-Aug-05, at 12:15, Tom Lane wrote: =?ISO-8859-1?Q?R=E9my_Beaumont?= [EMAIL PROTECTED] writes: The stats of the NetApp do confirm that it is sitting idle. Really? CPU NFS CIFS HTTP TotalNet kB/s Disk kB/s Tape kB/s Cache Cache CP CP Disk DAFS FCP iSCSI FCP kB/s in out read write read write age hit time ty util in out 2% 0 0 0 139 0 0 2788 0 0 0 3 96% 0% - 15% 0 139 0 3 2277 2% 0 0 0 144 0 0 2504 0 0 0 3 96% 0% - 18% 0 144 0 3 2150 2% 0 0 0 130 0 0 2212 0 0 0 3 96% 0% - 13% 0 130 0 3 1879 3% 0 0 0 169 0 0 2937 80 0 0 3 96% 0% - 13% 0 169 0 4 2718 2% 0 0 0 139 0 0 2448 0 0 0 3 96% 0% - 12% 0 139 0 3 2096 I know zip about NetApps, but doesn't the 8th column indicate pretty steady disk reads? Yes, but they are very low. At 15% usage, it's pretty much sitting idle if you consider that the OS reports that one of the processor is spending more then 80% of it's time in IOwait. Rémy regards, tom lane ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] High load and iowait but no disk access
On Mon, Aug 29, 2005 at 09:42:46AM -0400, Rémy Beaumont wrote: We have been trying to pinpoint what originally seem to be a I/O bottleneck but which now seems to be an issue with either Postgresql or RHES 3. Nope, it's an IO bottleneck. The behavior we see is that when running queries that do random reads on disk, IOWAIT goes over 80% and actual disk IO falls to a crawl at a throughput bellow 3000kB/s That's the sign of an IO bottleneck. The stats of the NetApp do confirm that it is sitting idle. Doing an strace on the Postgresql process shows that is it doing seeks and reads. So my question is where is this iowait time spent ? Waiting for the seeks. postgres doesn't do async io, so it requests a block, waits for it to come in, then requests another block, etc. The utilization on the netapp isn't going to be high because it doesn't have a queue of requests and can't do readahead because the IO is random. The only way to improve the situation would be to reduce the latency of the seeks. If I read the numbers right you're only getting about 130 seeks/s, which ain't great. I don't know how much latency the netapp adds in the this configuration; have you tried benchmarking direct-attach disks? Mike Stone ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PERFORM] High load and iowait but no disk access
=?ISO-8859-1?Q?R=E9my_Beaumont?= [EMAIL PROTECTED] writes: The stats of the NetApp do confirm that it is sitting idle. Really? CPU NFS CIFS HTTP TotalNet kB/s Disk kB/s Tape kB/s Cache Cache CP CP Disk DAFS FCP iSCSI FCP kB/s in out read write read write age hit time ty util in out 2% 0 0 0 139 0 0 2788 0 0 0 3 96% 0% - 15% 0 139 0 3 2277 2% 0 0 0 144 0 0 2504 0 0 0 3 96% 0% - 18% 0 144 0 3 2150 2% 0 0 0 130 0 0 2212 0 0 0 3 96% 0% - 13% 0 130 0 3 1879 3% 0 0 0 169 0 0 2937 80 0 0 3 96% 0% - 13% 0 169 0 4 2718 2% 0 0 0 139 0 0 2448 0 0 0 3 96% 0% - 12% 0 139 0 3 2096 I know zip about NetApps, but doesn't the 8th column indicate pretty steady disk reads? regards, tom lane ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] High load and iowait but no disk access
=?ISO-8859-1?Q?R=E9my_Beaumont?= [EMAIL PROTECTED] writes: On 30-Aug-05, at 12:15, Tom Lane wrote: I know zip about NetApps, but doesn't the 8th column indicate pretty steady disk reads? Yes, but they are very low. Sure, but that's more or less what you'd expect if the thing is randomly seeking all over the disk :-(. Just because it's a NetApp doesn't mean it's got zero seek time. You did not say what sort of query this is, but I gather that it's doing an indexscan on a table that is not at all in index order. Possible solutions involve reverting to a seqscan (have you forced the planner to choose an indexscan here, either directly or by lowering random_page_cost?) or CLUSTERing the table by the index (which would need to be repeated periodically, so it's not a great answer). regards, tom lane ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PERFORM] High load and iowait but no disk access
On 30-Aug-05, at 12:29, Tom Lane wrote: =?ISO-8859-1?Q?R=E9my_Beaumont?= [EMAIL PROTECTED] writes: On 30-Aug-05, at 12:15, Tom Lane wrote: I know zip about NetApps, but doesn't the 8th column indicate pretty steady disk reads? Yes, but they are very low. Sure, but that's more or less what you'd expect if the thing is randomly seeking all over the disk :-(. Just because it's a NetApp doesn't mean it's got zero seek time. Per NetApp, the disk utilization percentage they report does include seek time, not just read/write operations. NetApp has been involved in trying to figure out what is going on and their claim is that the NetApp filer is not IO bound. You did not say what sort of query this is, but I gather that it's doing an indexscan on a table that is not at all in index order. Yes, most of those queries are doing an indexscan. It's a fresh restore of our production database that we have vacuumed/analyzed. Possible solutions involve reverting to a seqscan (have you forced the planner to choose an indexscan here, either directly or by lowering random_page_cost?) No. or CLUSTERing the table by the index (which would need to be repeated periodically, so it's not a great answer). Will try to cluster the tables and see if it changes anything. Still doesn't explain what is going on with those seeks. Thanks, Rémy regards, tom lane ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PERFORM] High load and iowait but no disk access
Remy, The behavior we see is that when running queries that do random reads on disk, IOWAIT goes over 80% and actual disk IO falls to a crawl at a throughput bellow 3000kB/s (We usually average 4 kB/s to 8 kB/s on sequential read operations on the netapps) This seems pretty low for a NetApp -- you should be able to manage up to 180mb/s, if not higher. Are you sure it's configured correctly? -- --Josh Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] High load and iowait but no disk access
Have you tried a different kernel? We run with a netapp over NFS without any issues, but we have seen high IO-wait on other Dell boxes (running and not running postgres) and RHES 3. We have replaced a Dell PowerEdge 350 running RH 7.3 with a PE750 with more memory running RHES3 and it be bogged down with IO waits due to syslog messages writing to the disk, the old slower server could handle it fine. I don't know if it is a Dell thing or a RH kernel, but we try different kernels on our boxes to try to find one that works better. We have not found one that stands out over anotherconsistently but we have been moving away from Update 2 kernel (2.4.21-15.ELsmp) due to server lockup issues. Unfortunately we get the best disk throughput on our few remaining 7.3 boxes. Woody IGLASS Networks www.iglass.net From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Rémy BeaumontSent: Monday, August 29, 2005 9:43 AMTo: pgsql-performance@postgresql.orgSubject: [PERFORM] High load and iowait but no disk access We have been trying to pinpoint what originally seem to be a I/O bottleneck but which now seems to be an issue with either Postgresql or RHES 3.We have the following test environment on which we can reproduce the problem:1) Test System ADell 6650 Quad Xeon Pentium 48 Gig of RAMOS: RHES 3 update 2Storage: NetApp FAS270 connected using an FC card using 10 disks2) Test System BDell Dual Xeon Pentium III2 Gig o RAMOS: RHES 3 update 2Storage: NetApp FAS920 connected using an FC card using 28 disksOur Database size is around 30G. The behavior we see is that when running queries that do random reads on disk, IOWAIT goes over 80% and actual disk IO falls to a crawl at a throughput bellow 3000kB/s (We usually average 4 kB/s to 8 kB/s on sequential read operations on the netapps)The stats of the NetApp do confirm that it is sitting idle. Doing an strace on the Postgresql process shows that is it doing seeks and reads.So my question is where is this iowait time spent ?Is there a way to pinpoint the problem in more details ?We are able to reproduce this behavior with Postgresql 7.4.8 and 8.0.3I have included the output of top,vmstat,strace and systat from the Netapp from System B while running a single query that generates this behavior.Rémytop output:06:27:28 up 5 days, 16:59, 6 users, load average: 1.04, 1.30, 1.0172 processes: 71 sleeping, 1 running, 0 zombie, 0 stoppedCPU states: cpu user nice system irq softirq iowait idletotal 2.7% 0.0% 1.0% 0.1% 0.2% 46.0% 49.5%cpu00 0.2% 0.0% 0.2% 0.0% 0.2% 2.2% 97.2%cpu01 5.3% 0.0% 1.9% 0.3% 0.3% 89.8% 1.9%Mem: 2061696k av, 2043936k used, 17760k free, 0k shrd, 3916k buff1566332k actv, 296648k in_d, 30504k in_cSwap: 16771584k av, 21552k used, 16750032k free 1933772k cachedPID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND30960 postgres 15 0 13424 10M 9908 D 2.7 0.5 2:00 1 postmaster30538 root 15 0 1080 764 524 S 0.7 0.0 0:43 0 sshd1 root 15 0 496 456 436 S 0.0 0.0 0:08 0 init2 root RT 0 0 0 0 SW 0.0 0.0 0:00 0 migration/03 root RT 0 0 0 0 SW 0.0 0.0 0:00 1 migration/14 root 15 0 0 0 0 SW 0.0 0.0 0:01 0 keventd5 root 34 19 0 0 0 SWN 0.0 0.0 0:00 0 ksoftirqd/06 root 34 19 0 0 0 SWN 0.0 0.0 0:00 1 ksoftirqd/19 root 15 0 0 0 0 SW 0.0 0.0 0:24 1 bdflush7 root 15 0 0 0 0 SW 0.0 0.0 6:53 1 kswapd8 root 15 0 0 0 0 SW 0.0 0.0 8:44 1 kscand10 root 15 0 0 0 0 SW 0.0 0.0 0:13 0 kupdated11 root 25 0 0 0 0 SW 0.0 0.0 0:00 0 mdrecoveryd17 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 ahc_dv_0vmstat output procs memory swap io system cpur b swpd free buff cache si so bi bo in cs us sy id wa0 1 21552 17796 4872 1931928 2 3 3 1 27 6 2 1 7 30 1 21552 18044 4880 1931652 0 0 1652 0 397 512 1 2 50 470 1 21552 17976 4896 1931664 0 0 2468 0 407 552 2 2 50 471 0 21552 17984 4896 1931608 0 0 2124 0 418 538 3 3 48 460 1 21552 18028 4900 1931536 0 0 1592 0 385 509 1 3 50 460 1 21552 18040 4916 1931488 0 0 1620 820 419 581 2 2 50 460 1 21552 17968 4916 1931536 0 4 1708 4 402 554 3 1 50 461 1 21552 18052 4916 1931388 0 0 1772 0 409 531 3 1 49 470 1 21552 17912 4924 1931492 0 0 1772 0 408 565 3 1 48 480 1 21552 17932 4932 1931440 0 4 1356 4 391 545 5 0 49 460 1 21552 18320 4944 1931016 0 4 1500 840 414 571 1 1 48 500 1 21552 17872 4944 1931440 0 0 2116 0 392 496 1 5 46 480 1 21552 18060 4944 1931232 0 0 2232 0 423 597 1 2 48 491 1 21552 17684 4944 1931584 0 0 1752 0 395 537 1 1 50 480 1 21552 18000 4944 1931240 0 0 1576 0 401 549 0 1 50 49NetApp stats:CPU NFS CIFS HTTP Total Net kB/s Disk kB/s Tape kB/s Cache Cache CP CP Disk DAFS FCP iSCSI FCP kB/sin out read write read write age hit time ty util in out2% 0 0 0 139 0 0 2788 0 0 0 3 96% 0% - 15% 0 139 0 3 22772% 0 0 0 144 0 0 2504 0 0 0 3 96% 0% - 18% 0 144 0 3 21502% 0 0 0 130 0 0 2212 0 0 0 3 96% 0% - 13% 0 130 0 3 18793% 0 0 0 169 0 0 2937 80 0 0 3 96% 0% - 13% 0 169 0 4 27182% 0 0 0 139 0 0 2448 0 0 0 3 96% 0% - 12% 0 139 0 3 20962% 0 0 0 137 0 0 2116 0 0 0 3 96% 0% - 10% 0
Re: [PERFORM] High load and iowait but no disk access
On 30-Aug-05, at 14:32, Josh Berkus wrote: Remy, The behavior we see is that when running queries that do random reads on disk, IOWAIT goes over 80% and actual disk IO falls to a crawl at a throughput bellow 3000kB/s (We usually average 4 kB/s to 8 kB/s on sequential read operations on the netapps) This seems pretty low for a NetApp -- you should be able to manage up to 180mb/s, if not higher. Are you sure it's configured correctly? Hi Josh, The config has been reviewed by NetApp. We do get rates higher then 80mb/s, but on average, that's what we get. Do you have NetApp filers deployed ? How many spindles do you have in your volume ? On which OS are you running Postgres ? Thanks, Rémy -- --Josh Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PERFORM] High load and iowait but no disk access
On 30-Aug-05, at 14:46, Anjan Dave wrote: I have seen references of changing the kernel io scheduler at boot time…not sure if it applies to RHEL3.0, or will help, but try setting ‘elevator=deadline’ during boot time or via grub.conf. That's only for RHEL 4.0. Have you tried running a simple ‘dd’ on the LUN? We get amazing performance using dd. The drives are in RAID10 configuration, right? NetApp has their own type of raid format (RAID4 aka WAFL) Rémy Thanks, Anjan From: Woody Woodring [mailto:[EMAIL PROTECTED] Sent: Tuesday, August 30, 2005 2:30 PM To: 'Rémy Beaumont'; pgsql-performance@postgresql.org Subject: Re: [PERFORM] High load and iowait but no disk access Have you tried a different kernel? We run with a netapp over NFS without any issues, but we have seen high IO-wait on other Dell boxes (running and not running postgres) and RHES 3. We have replaced a Dell PowerEdge 350 running RH 7.3 with a PE750 with more memory running RHES3 and it be bogged down with IO waits due to syslog messages writing to the disk, the old slower server could handle it fine. I don't know if it is a Dell thing or a RH kernel, but we try different kernels on our boxes to try to find one that works better. We have not found one that stands out over another consistently but we have been moving away from Update 2 kernel (2.4.21-15.ELsmp) due to server lockup issues. Unfortunately we get the best disk throughput on our few remaining 7.3 boxes. Woody IGLASS Networks www.iglass.net From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Rémy Beaumont Sent: Monday, August 29, 2005 9:43 AM To: pgsql-performance@postgresql.org Subject: [PERFORM] High load and iowait but no disk access We have been trying to pinpoint what originally seem to be a I/O bottleneck but which now seems to be an issue with either Postgresql or RHES 3. We have the following test environment on which we can reproduce the problem: 1) Test System A Dell 6650 Quad Xeon Pentium 4 8 Gig of RAM OS: RHES 3 update 2 Storage: NetApp FAS270 connected using an FC card using 10 disks 2) Test System B Dell Dual Xeon Pentium III 2 Gig o RAM OS: RHES 3 update 2 Storage: NetApp FAS920 connected using an FC card using 28 disks Our Database size is around 30G. The behavior we see is that when running queries that do random reads on disk, IOWAIT goes over 80% and actual disk IO falls to a crawl at a throughput bellow 3000kB/s (We usually average 4 kB/s to 8 kB/s on sequential read operations on the netapps) The stats of the NetApp do confirm that it is sitting idle. Doing an strace on the Postgresql process shows that is it doing seeks and reads. So my question is where is this iowait time spent ? Is there a way to pinpoint the problem in more details ? We are able to reproduce this behavior with Postgresql 7.4.8 and 8.0.3 I have included the output of top,vmstat,strace and systat from the Netapp from System B while running a single query that generates this behavior. Rémy top output: 06:27:28 up 5 days, 16:59, 6 users, load average: 1.04, 1.30, 1.01 72 processes: 71 sleeping, 1 running, 0 zombie, 0 stopped CPU states: cpu user nice system irq softirq iowait idle total 2.7% 0.0% 1.0% 0.1% 0.2% 46.0% 49.5% cpu00 0.2% 0.0% 0.2% 0.0% 0.2% 2.2% 97.2% cpu01 5.3% 0.0% 1.9% 0.3% 0.3% 89.8% 1.9% Mem: 2061696k av, 2043936k used, 17760k free, 0k shrd, 3916k buff 1566332k actv, 296648k in_d, 30504k in_c Swap: 16771584k av, 21552k used, 16750032k free 1933772k cached PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND 30960 postgres 15 0 13424 10M 9908 D 2.7 0.5 2:00 1 postmaster 30538 root 15 0 1080 764 524 S 0.7 0.0 0:43 0 sshd 1 root 15 0 496 456 436 S 0.0 0.0 0:08 0 init 2 root RT 0 0 0 0 SW 0.0 0.0 0:00 0 migration/0 3 root RT 0 0 0 0 SW 0.0 0.0 0:00 1 migration/1 4 root 15 0 0 0 0 SW 0.0 0.0 0:01 0 keventd 5 root 34 19 0 0 0 SWN 0.0 0.0 0:00 0 ksoftirqd/0 6 root 34 19 0 0 0 SWN 0.0 0.0 0:00 1 ksoftirqd/1 9 root 15 0 0 0 0 SW 0.0 0.0 0:24 1 bdflush 7 root 15 0 0 0 0 SW 0.0 0.0 6:53 1 kswapd 8 root 15 0 0 0 0 SW 0.0 0.0 8:44 1 kscand 10 root 15 0 0 0 0 SW 0.0 0.0 0:13 0 kupdated 11 root 25 0 0 0 0 SW 0.0 0.0 0:00 0 mdrecoveryd 17 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 ahc_dv_0 vmstat output procs memory swap io system cpu r b swpd free buff cache si so bi bo in cs us sy id wa 0 1 21552 17796 4872 1931928 2 3 3 1 27 6 2 1 7 3 0 1 21552 18044 4880 1931652 0 0 1652 0 397 512 1 2 50 47 0 1 21552 17976 4896 1931664 0 0 2468 0 407 552 2 2 50 47 1 0 21552 17984 4896 1931608 0 0 2124 0 418 538 3 3 48 46 0 1 21552 18028 4900 1931536 0 0 1592 0 385 509 1 3 50 46 0 1 21552 18040 4916 1931488 0 0 1620 820 419 581 2 2 50 46 0 1 21552 17968 4916 1931536 0 4 1708 4 402 554 3 1 50 46 1 1 21552 18052 4916 1931388 0 0 1772 0 409 531 3 1 49 47 0 1 21552 17912 4924 1931492 0 0 1772 0 408 565 3 1 48 48 0 1 21552 17932 4932 1931440 0 4 1356 4 391 545 5 0
Re: [PERFORM] High load and iowait but no disk access
I have seen references of changing the kernel io scheduler at boot timenot sure if it applies to RHEL3.0, or will help, but try setting elevator=deadline during boot time or via grub.conf. Have you tried running a simple dd on the LUN? The drives are in RAID10 configuration, right? Thanks, Anjan From: Woody Woodring [mailto:[EMAIL PROTECTED] Sent: Tuesday, August 30, 2005 2:30 PM To: 'Rémy Beaumont'; pgsql-performance@postgresql.org Subject: Re: [PERFORM] High load and iowait but no disk access Have you tried a different kernel? We run with a netapp over NFS without any issues, but we have seen high IO-wait on other Dell boxes (running and not running postgres) and RHES 3. We have replaced a Dell PowerEdge 350 running RH 7.3 with a PE750 with more memory running RHES3 and it be bogged down with IO waits due to syslog messages writing to the disk, the old slower server could handle it fine. I don't know if it is a Dell thing or a RH kernel, but we try different kernels on our boxes to try to find one that works better. We have not found one that stands out over anotherconsistently but we have been moving away from Update 2 kernel (2.4.21-15.ELsmp) due to server lockup issues. Unfortunately we get the best disk throughput on our few remaining 7.3 boxes. Woody IGLASS Networks www.iglass.net From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Rémy Beaumont Sent: Monday, August 29, 2005 9:43 AM To: pgsql-performance@postgresql.org Subject: [PERFORM] High load and iowait but no disk access We have been trying to pinpoint what originally seem to be a I/O bottleneck but which now seems to be an issue with either Postgresql or RHES 3. We have the following test environment on which we can reproduce the problem: 1) Test System A Dell 6650 Quad Xeon Pentium 4 8 Gig of RAM OS: RHES 3 update 2 Storage: NetApp FAS270 connected using an FC card using 10 disks 2) Test System B Dell Dual Xeon Pentium III 2 Gig o RAM OS: RHES 3 update 2 Storage: NetApp FAS920 connected using an FC card using 28 disks Our Database size is around 30G. The behavior we see is that when running queries that do random reads on disk, IOWAIT goes over 80% and actual disk IO falls to a crawl at a throughput bellow 3000kB/s (We usually average 4 kB/s to 8 kB/s on sequential read operations on the netapps) The stats of the NetApp do confirm that it is sitting idle. Doing an strace on the Postgresql process shows that is it doing seeks and reads. So my question is where is this iowait time spent ? Is there a way to pinpoint the problem in more details ? We are able to reproduce this behavior with Postgresql 7.4.8 and 8.0.3 I have included the output of top,vmstat,strace and systat from the Netapp from System B while running a single query that generates this behavior. Rémy top output: 06:27:28 up 5 days, 16:59, 6 users, load average: 1.04, 1.30, 1.01 72 processes: 71 sleeping, 1 running, 0 zombie, 0 stopped CPU states: cpu user nice system irq softirq iowait idle total 2.7% 0.0% 1.0% 0.1% 0.2% 46.0% 49.5% cpu00 0.2% 0.0% 0.2% 0.0% 0.2% 2.2% 97.2% cpu01 5.3% 0.0% 1.9% 0.3% 0.3% 89.8% 1.9% Mem: 2061696k av, 2043936k used, 17760k free, 0k shrd, 3916k buff 1566332k actv, 296648k in_d, 30504k in_c Swap: 16771584k av, 21552k used, 16750032k free 1933772k cached PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND 30960 postgres 15 0 13424 10M 9908 D 2.7 0.5 2:00 1 postmaster 30538 root 15 0 1080 764 524 S 0.7 0.0 0:43 0 sshd 1 root 15 0 496 456 436 S 0.0 0.0 0:08 0 init 2 root RT 0 0 0 0 SW 0.0 0.0 0:00 0 migration/0 3 root RT 0 0 0 0 SW 0.0 0.0 0:00 1 migration/1 4 root 15 0 0 0 0 SW 0.0 0.0 0:01 0 keventd 5 root 34 19 0 0 0 SWN 0.0 0.0 0:00 0 ksoftirqd/0 6 root 34 19 0 0 0 SWN 0.0 0.0 0:00 1 ksoftirqd/1 9 root 15 0 0 0 0 SW 0.0 0.0 0:24 1 bdflush 7 root 15 0 0 0 0 SW 0.0 0.0 6:53 1 kswapd 8 root 15 0 0 0 0 SW 0.0 0.0 8:44 1 kscand 10 root 15 0 0 0 0 SW 0.0 0.0 0:13 0 kupdated 11 root 25 0 0 0 0 SW 0.0 0.0 0:00 0 mdrecoveryd 17 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 ahc_dv_0 vmstat output procs memory swap io system cpu r b swpd free buff cache si so bi bo in cs us sy id wa 0 1 21552 17796 4872 1931928 2 3 3 1 27 6 2 1 7 3 0 1 21552 18044 4880 1931652 0 0 1652 0 397 512 1 2 50 47 0 1 21552 17976 4896 1931664 0 0 2468 0 407 552 2 2 50 47 1 0 21552 17984 4896 1931608 0 0 2124 0 418 538 3 3 48 46 0 1 21552 18028 4900 1931536 0 0 1592 0 385 509 1 3 50 46 0 1 21552 18040 4916 1931488 0 0 1620 820 419 581 2 2 50 46 0 1 21552 17968 4916 1931536 0 4 1708 4 402 554 3 1 50 46 1 1 21552 18052 4916 1931388 0 0 1772 0 409 531 3 1 49 47 0 1 21552 17912 4924 1931492 0 0 1772 0 408 565 3 1 48 48 0 1 21552 17932 4932 1931440 0 4 1356 4 391 545 5 0 49 46 0 1 21552 18320 4944 1931016 0 4 1500 840 414 571 1 1 48 50 0 1 21552 17872 4944 1931440 0 0 2116 0 392 496 1 5 46 48 0 1 21552 18060 4944 1931232 0 0 2232 0 423 597 1 2 48 49 1 1 21552 17684 4944 1931584 0 0 1752 0 395 537 1 1 50 48 0 1
Re: [PERFORM] High load and iowait but no disk access
This might be optimal behavior from the hardware. Random reads are hard to optimize for--except if you have enough physical memory to hold the entire dataset. Cached reads (either in array controller or OS buffer cache) should return nearly immediately. But random reads probably aren't cached. And any read-ahead alogorithms or other types of performance enhancements in the hardware or OS go out the window--because the behavior isn't predictable. Each time a drive spindle needs to move to a new track, it requires at least a couple of miliseconds. Sequential reads only require this movement infrequently. But random reads may be forcing this movement for every IO operation. Since the bottleneck in random reads is the physical hard drives themselves, everything else stands around waiting. Fancy hardware can optimize everything else -- writes with write cache, sequential reads with read-ahead and read cache. But there's no real solution to a purely random read workload except perhaps creating different disk groups to help avoid spindle contention. I like this tool: http://www.soliddata.com/products/iotest.html It allows you to select pure workloads (read/write/sequential/random), and it runs against raw devices, so you bypass the OS buffer cache. When I've run it I've always seen sequential activity get much much higher throughput than random. Quoting Anjan Dave [EMAIL PROTECTED]: I have seen references of changing the kernel io scheduler at boot time...not sure if it applies to RHEL3.0, or will help, but try setting 'elevator=deadline' during boot time or via grub.conf. Have you tried running a simple 'dd' on the LUN? The drives are in RAID10 configuration, right? Thanks, Anjan _ From: Woody Woodring [mailto:[EMAIL PROTECTED] Sent: Tuesday, August 30, 2005 2:30 PM To: 'Rémy Beaumont'; pgsql-performance@postgresql.org Subject: Re: [PERFORM] High load and iowait but no disk access Have you tried a different kernel? We run with a netapp over NFS without any issues, but we have seen high IO-wait on other Dell boxes (running and not running postgres) and RHES 3. We have replaced a Dell PowerEdge 350 running RH 7.3 with a PE750 with more memory running RHES3 and it be bogged down with IO waits due to syslog messages writing to the disk, the old slower server could handle it fine. I don't know if it is a Dell thing or a RH kernel, but we try different kernels on our boxes to try to find one that works better. We have not found one that stands out over another consistently but we have been moving away from Update 2 kernel (2.4.21-15.ELsmp) due to server lockup issues. Unfortunately we get the best disk throughput on our few remaining 7.3 boxes. Woody IGLASS Networks www.iglass.net _ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Rémy Beaumont Sent: Monday, August 29, 2005 9:43 AM To: pgsql-performance@postgresql.org Subject: [PERFORM] High load and iowait but no disk access We have been trying to pinpoint what originally seem to be a I/O bottleneck but which now seems to be an issue with either Postgresql or RHES 3. We have the following test environment on which we can reproduce the problem: 1) Test System A Dell 6650 Quad Xeon Pentium 4 8 Gig of RAM OS: RHES 3 update 2 Storage: NetApp FAS270 connected using an FC card using 10 disks 2) Test System B Dell Dual Xeon Pentium III 2 Gig o RAM OS: RHES 3 update 2 Storage: NetApp FAS920 connected using an FC card using 28 disks Our Database size is around 30G. The behavior we see is that when running queries that do random reads on disk, IOWAIT goes over 80% and actual disk IO falls to a crawl at a throughput bellow 3000kB/s (We usually average 4 kB/s to 8 kB/s on sequential read operations on the netapps) The stats of the NetApp do confirm that it is sitting idle. Doing an strace on the Postgresql process shows that is it doing seeks and reads. So my question is where is this iowait time spent ? Is there a way to pinpoint the problem in more details ? We are able to reproduce this behavior with Postgresql 7.4.8 and 8.0.3 I have included the output of top,vmstat,strace and systat from the Netapp from System B while running a single query that generates this behavior. Rémy top output: 06:27:28 up 5 days, 16:59, 6 users, load average: 1.04, 1.30, 1.01 72 processes: 71 sleeping, 1 running, 0 zombie, 0 stopped CPU states: cpu user nice system irq softirq iowait idle total 2.7% 0.0% 1.0% 0.1% 0.2% 46.0% 49.5% cpu00 0.2% 0.0% 0.2% 0.0% 0.2% 2.2% 97.2% cpu01 5.3% 0.0% 1.9% 0.3% 0.3% 89.8% 1.9% Mem: 2061696k av, 2043936k used, 17760k free, 0k shrd, 3916k buff 1566332k actv, 296648k in_d, 30504k in_c Swap: 16771584k av, 21552k used, 16750032k free 1933772k cached PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND 30960 postgres 15 0 13424 10M 9908 D 2.7 0.5 2:00 1
