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 For the SATA: # cat /sys/block/sdb/queue/scheduler noop anticipatory deadline [cfq] Then in the VM: # cat /sys/block/xvda/queue/scheduler [noop] anticipatory deadline cfq Ron -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
