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). -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
