> I'll run a bonnie++ first. As the CPUs seem to be idle most of the time
> (see the vmstat.out below), I'm suspecting the RAID or disks.
You have a huge amount of iowait !
Did you put the xlog on a separate disk ?
What filesystem do you use ?
Did you check that your BBU cache works ?
For that run a dumb script which does INSERTS in a test table in
autocommit mode ; if you get (7200rpm / 60) = 120 inserts / sec or less,
the good news is that your drives don't lie about fsync, the bad news is
that your BBU cache isn't working...