Re: Having I/O problems in simple virtualized environment

On Sun, Jan 29, 2012 at 6:18 PM, Ron Arts <ron(dot)arts(at)gmail(dot)com> wrote:
> Hi list,
>
> I am running PostgreSQL 8.1 (CentOS 5.7) on a VM on a single XCP (Xenserver) host.
> This is a HP server with 8GB, Dual Quad Core, and 2 SATA in RAID-1.
>
> The problem is: it's running very slow compared to running it on bare metal, and
> the VM is starving for I/O bandwidht, so other processes (slow to a crawl.
> This does not happen on bare metal.
>
> I had to replace the server with a bare-metal one, I could not troubleshoot in production.
> Also it was hard to emulte the workload for that VM in a test environment, so I
> concentrated on PostgreSQLand why it apparently generated so much I/O.
>
> Before I start I should confess having only spotty experience with Xen and PostgreSQL
> performance testing.
>
> I setup a test Xen server created a CentOS5.7 VM with out-of-the-box PostgreSQL and ran:
> pgbench -i  pgbench ; time pgbench -t 100000 pgbench
> This ran for 3:28. Then I replaced the SATA HD with an SSD disk, and reran the test.
> It ran for 2:46. This seemed strange as I expected the run to finish much faster.
>
> I reran the first test on the SATA, and looked at CPU and I/O use. The CPU was not used
> too much in both the VM (30%) and in dom0 (10%). The I/O use was not much as well,
> around 8MB/sec in the VM. (Couldn't use iotop in dom0, because of missing kernel support
> in XCP 1.1).
>
> It reran the second test on SSD, and experienced almost the same CPU, and I/O load.
>
> (I now probably need to run the same test on bare metal, but didn't get to that yet,
> all this already ruined my weekend.)
>
> Now I came this far, can anybody give me some pointers? Why doesn't pgbench saturate
> either the CPU or the I/O? Why does using SSD only change the performance this much?

Ok, one point: Which IO scheduler are you using? (on dom0 and on the VM).