I am wondering if your wait is caused by contention between pg_autovacuum and the DELETE that is running. Your large Pg blocksize (32K) *may* be contributing to any possible contention as well. Maybe try disabling pg_autovacuum to see if there is any change in behaviour.

Also going through my head is '32 Kb bock's size (to match ffs and raid block's size)' - does that mean you have raid strip size = 32K? maybe try 128K (I know it sounds like a bad thing, but generally raid stripes of 128K->256K are better than 32K->64K)



Pailloncy Jean-Gérard wrote:


I found the same problem.

I use OpenBSD 3.3,
On Pentium 2,4 GHz with 1 Gb RAM, RAID 10.
With PostgreSQL 7.4.1 with 32 Kb bock's size (to match ffs and raid block's size)
With pg_autovacuum daemon from Pg 7.5.

I run a web indexer.
sd0 raid-1 with system pg-log and indexer-log
sd1 raid-10 with pg-data and indexer-data
The sd1 disk achives between 10 and 40 Mb/s on normal operation.

When I get semwait in top, system waits ;-)
Not much disk activity.
Not much log in pg or indexer.
Just wait....

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