On Wed, May 25, 2011 at 01:37:47PM +0100, Simon Riggs wrote: > > That's the way SQLServer and Oracle work, but not PostgreSQL. We can > clear down WAL files even during a long running transaction. > > For us, "unneeded" means prior to the second-to-last checkpoint record.
Well, they're obviously not getting cleared down, so they must be needed. I know how Postgres is supposed to work in these cases, but in my experience you cannot rely on the OP's calculation to provide you with a true maximum. Pathological conditions result in a lot of WAL segments hanging around. What I really suspect is that this has to do with the way I/O scheduling works, particularly in the presence of the bgwriter. But I don't feel comfortable suggesting particular reasons for what I've experienced in production. What I _can_ tell you is that, when I've had to do large restores like this, I wanted plenty of overhead for WAL. ISTR dedicating 40G to WAL one time for a case like this. A -- Andrew Sullivan a...@crankycanuck.ca -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
