On Mon, Jan 30, 2012 at 3:11 AM, Ron Arts <ron.arts@xxxxxxxxx> wrote: > Op 30-01-12 02:52, Jose Ildefonso Camargo Tolosa schreef: >> On Sun, Jan 29, 2012 at 6:18 PM, Ron Arts <ron.arts@xxxxxxxxx> 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). > > Ok, first dom0: > > For the SSD (hda): > # cat /sys/block/sda/queue/scheduler > [noop] anticipatory deadline cfq Use deadline. > > For the SATA: > # cat /sys/block/sdb/queue/scheduler > noop anticipatory deadline [cfq] Use deadline too (this is specially true if sdb is a raid array). > > Then in the VM: > > # cat /sys/block/xvda/queue/scheduler > [noop] anticipatory deadline cfq Should be ok for the VM. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
