Folks, In the past, setting vacuum_freeze_min_age (vfma) really low (say to 10000 or 50000) would have caused lots of extra writing work due to dirtying extra pages for freezing. This has been our stated reason to keep vfma high, despite the obvious advantage of freezing tuples while they're still in the cache. With the visibility map, though, vfma should only be dirtying pages which vacuum is already visiting because there's dirty tuples on the page. That is, pages which vacuum will probably dirty anyway, freezing or not. (This is assuming one has applied the 9.2.3 update.) Given that, it seems like the cost of lowering vfma *should* be marginal. The only extra work done by a lower vfma should be: 1. extra cpu time to put in the froxenXIDs on vacuumed pages, and 2. dirtying the minority of pages which vacuum decided to scan, but not write to. The second point is the one where I'm not sure how to evaluate. How likely, as of 9.2, is vacuum to visit a page and not dirty it? And are there other costs I'm not thinking of? -- Josh Berkus PostgreSQL Experts Inc. http://pgexperts.com -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
