On Jul 9, 2012, at 10:52 PM, Jeff Janes wrote: > On Mon, Jul 9, 2012 at 10:39 PM, David Kerr <dmk@xxxxxxxxxxxxxx> wrote: >> >> I thought that the idea of checkpoint_completion_target was that we try to >> finish writing >> out the data throughout the entire checkpoint (leaving some room to spare, >> in my case 30% >> of the total estimated checkpoint time) >> >> But what appears to be happening is that all of the data is being written >> out at the end of the checkpoint. > > Postgres is writing data out to the kernel throughout the checkpoint. > But the kernel is just buffering it up dirty, until the end of the > checkpoint when the fsyncs start landing like bombs. Ahh. duh! I guess i assumed that the point of spreading the checkpoint I/O was spreading the syncs out. > >> >> This happens at every checkpoint while the system is under load. >> >> I get the feeling that this isn't the correct behavior and i've done >> something wrong. >> >> Also, I didn't see this sort of behavior in PG 8.3, however unfortunately, I >> don't have data to back that >> statement up. > > Did you have less RAM back when you were running PG 8.3? nope. I was on RHEL 5.5 back then though. > >> Any suggestions. I'm willing and able to profile, or whatever. > > Who much RAM do you have? What are your settings for /proc/sys/vm/dirty_* ? 256G and I've been running with this for a while now, but I think that's the default in RHEL 6+ echo 10 > /proc/sys/vm/dirty_ratio echo 5 >/proc/sys/vm/dirty_background_ratio -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
