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 ;)
Or more to the point here, if each random read is 4MB long. Which makes it more like sequential reads, randomly-piecewise, rather than random reads.
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.
I wouldn't be sure. He is using an iodepth of 256 in his benchmark. It wouldn't be all that outrageous for a disk to be able to find 3 or 4 sectors per revolution it can read, when it has that many to choose from.
