Re: Very poor read performance, query independent

On Mon, Jul 10, 2017 at 9:03 AM, Charles Nadeau
<charles(dot)nadeau(at)gmail(dot)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; your benchmark is
lying to you. Random reads on spinning media are absolutely going to
be storage bound.

merlin