Transaction wraparound vacuum synchronicity

On Wed, Mar 09, 2011 at 10:52:25AM +0000, Michael Graham wrote:

Perhaps I should just force a vacuum on some of the tables the break it?

It seems to me that you might want to VACUUM FREEZE the table when it
becomes read-only. (I note that FREEZE is deprecated according to the
9.0 manual. Too bad, that, because it seems to me that this is a use
case where one might want to put a thumb on the scale, and having to
twiddle a parameter just to affect one table is kind of
user-unfriendly.)

A

--
Andrew Sullivan
ajs@crankycanuck.ca

On Wed, Mar 09, 2011 at 10:52:25AM +0000, Michael Graham wrote:

I have a database with a number of tables that are partitioned monthly,
after that the tables are mostly read only (on rare occasions we may
delete from a table but normally we just drop the partitions). Recently
I've noticed that we have a lot of these tables are vacuumed around the
same time, after a little big of digging I've realised that postgres is
vacuuming them to stop xaction wrap around. So for example in a few
million xactions (later today) postgres is going to want to vacuum 37
tables for just this reason.

I know I can fiddle autovacuum_freeze_max_age and vacuum_freeze_min_age
to change how regularly the tables have this occur, and I can do this on
a per table basis in pg_autovacuum (yes this means I'm running an old
version, version 8.2) but what I'm wondering is how other people are
breaking this synchronisation?

Should I add a random value to the freeze_max_age for all the old tables
when I start a new month?

I haven't tried such a thing, but that seems like a great plan. You wouldn't
need to change all old tables every month or actually make it random. Each time
you create a monthly table, decrease the last-used-value by some fixed interval
and use that for the new table. (You'll probably need to subtract, not add,
because per-table freeze_max_age cannot usefully exceed the global
autovacuum_freeze_max_age.)

Or do the same with the freeze_min_age?

This should work about as well, perhaps slightly better. Reducing
freeze_max_age can force freeze_min_age down, but not vice-versa.

Perhaps I should just force a vacuum on some of the tables the break it?

That strategy does work. However, if you ever run a full-database manual VACUUM
(in PostgreSQL < 8.4, anyway), they'd get resynchronized. Fiddling the
per-table freeze_max_age or freeze_min_age does not have that liability.

nm