Carlos,
> We've migrated one of our servers from pg 7.4 to 8.1 and from times to
> times (4 hours) the server start doing a lot of context switching and all
> transactions become very slow.
>
> The average context switching for this server as vmstat shows is 1 but when
> the problem occurs it goes to 250000.
Context Switching is a symptom rather than a cause. What's most likely
happening is that you have a combined heavy-CPU and heavy-IO workload, so you
have bursts of CPU activity stalled by iowaits.
Can you check the rate of iowaits during the "storm" periods?
Also, is this Xeon? And are you saying that you *didn't* have this issue
under 7.4?
--
Josh Berkus
PostgreSQL @ Sun
San Francisco