On Mon, Aug 12, 2013 at 11:47 PM, Jeff Janes <jeff.ja...@gmail.com> wrote: > Reviving a very old thread, because I've run into the issue again. > On Tue, May 29, 2012 at 11:58 AM, Robert Haas <robertmh...@gmail.com> wrote: >> On Fri, May 25, 2012 at 4:06 PM, Jeff Janes <jeff.ja...@gmail.com> wrote: >>> If I invoke vacuum manually and do so with VacuumCostDelay == 0, I >>> have basically declared my intentions to get this pain over with as >>> fast as possible even if it might interfere with other processes. >>> >>> Under that condition, shouldn't it use BAS_BULKWRITE rather than >>> BAS_VACUUM? The smaller ring size leads to a lot of synchronous WAL >>> flushes which I think can slow the vacuum down a lot. >> >> Of course, an autovacuum of a really big table could run too slowly, >> too, even though it's not a foreground task. > > True. But almost by definition, an autovacuum is not trying to run > inside a maintenance window. > > Would it be reasonable to upgrade the ring buffer size whenever > VacuumCostDelay is zero, regardless of whether it is a manual or an > auto vac? One thing I worry about is that many people may have > changed autovacuum_vacuum_cost_delay from 20 directly to 0 or -1, and > the accidental throttling on WAL syncs might be the only thing > preventing their system from falling over each time autovac of a large > table kicks in.
I'm not sure what the right thing to do here is, but I definitely agree there's a problem. There are definitely cases where people want or indeed need to vacuum as fast as possible, and using a small ring buffer is not the way to do that. Now, tying that to VacuumCostDelay doesn't seem right, because setting that to 0 shouldn't suddenly change the behavior in other ways, as well. In general, the approach we've taken so far has been to try to hide the ring-buffer behavior from users and not make it tunable, but I'm not sure we can really get away with that in this case. Increasing the ring-buffer size has system-wide performance implications which could be very good (less bloat) or very bad (I/O starvation of concurrent activity). I don't think the system knows enough to guess which one will be better in any particular case. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers