Joshua, I use noop as the scheduler because it is better to let the RAID controller re-arrange the IO operation before they reach the disk. Read ahead is set to 128:
charles@hpdl380g6:~$ cat /sys/block/sdc/queue/read_ahead_kb 128 charles@hpdl380g6:~$ cat /sys/block/sdc/queue/scheduler [noop] deadline cfq Thanks! Charles On Wed, Jul 12, 2017 at 1:42 AM, Joshua D. Drake <j...@commandprompt.com> wrote: > On 07/11/2017 04:15 PM, Merlin Moncure wrote: > >> On Mon, Jul 10, 2017 at 9:03 AM, Charles Nadeau >> <charles.nad...@gmail.com> wrote: >> >>> I’m running PostgreSQL 9.6.3 on Ubuntu 16.10 (kernel 4.4.0-85-generic). >>> Hardware is: >>> >>> *2x Intel Xeon E5550 >>> >>> *72GB RAM >>> >>> *Hardware RAID10 (4 x 146GB SAS 10k) P410i controller with 1GB FBWC (80% >>> read/20% write) for Postgresql data only: >>> >>> The problem I have is very poor read. When I benchmark my array with fio >>> I >>> get random reads of about 200MB/s and 1100IOPS and sequential reads of >>> about >>> 286MB/s and 21000IPS. But when I watch my queries using pg_activity, I >>> get >>> at best 4MB/s. Also using dstat I can see that iowait time is at about >>> 25%. >>> This problem is not query-dependent. >>> >> >> Stop right there. 1100 iops * 8kb = ~8mb/sec raw which might >> reasonably translate to 4mb/sec to the client. 200mb/sec random >> read/sec on spinning media is simply not plausible; >> > > Sure it is, if he had more than 4 disks ;) but he also isn't going to get > 1100 IOPS from 4 10k disks. The average 10k disk is going to get around 130 > IOPS . If he only has 4 then there is no way he is getting 1100 IOPS. > > Using the above specs (4x146GB) the best he can reasonably hope for from > the drives themselves is about 50MB/s add in the 1GB FWBC and that is how > he is getting those high numbers for IOPS but that is because of caching. > > He may need to adjust his readahead as well as his kernel scheduler. At a > minimum he should be able to saturate the drives without issue. > > JD > > > > -- > Command Prompt, Inc. || http://the.postgres.company/ || @cmdpromptinc > > PostgreSQL Centered full stack support, consulting and development. > Advocate: @amplifypostgres || Learn: https://pgconf.us > ***** Unless otherwise stated, opinions are my own. ***** > -- Charles Nadeau Ph.D. http://charlesnadeau.blogspot.com/
