On Tue, Jul 11, 2017 at 4:42 PM, 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 ;) 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. Cheers, Jeff
