On 2013-01-30 05:39:29 -0800, Kevin Grittner wrote: > Andres Freund <and...@2ndquadrant.com> wrote: > > On 2013-01-29 16:09:52 +1100, Josh Berkus wrote: > >> > >>> I have to admit, I fail to see why this is a good idea. There > >>> isn't much of an efficiency bonus in freezing early (due to > >>> hint bits) and vacuums over vacuum_freeze_table_age are > >>> considerably more expensive as they have to scan the whole heap > >>> instead of using the visibilitymap. And if you don't vacuum the > >>> whole heap you can't lower relfrozenxid. So changing > >>> freeze_min_age doesn't help at all to avoid anti-wraparound > >>> vacuums. > >>> > >>> Am I missing something? > >> > >> Yep. First, you're confusing vacuum_freeze_table_age and > >> vacuum_freeze_min_age. > > > > Don't think I did. I was talking about vacuum_freeze_table_age > > because that influences the amount of full-table scans > > Not any more than vacuum_freeze_min_age does.
Well, vacuum_freeze_min_age is 50m while vacuum_freeze_table_age is 150m. > http://www.postgresql.org/docs/9.2/interactive/routine-vacuuming.html#AUTOVACUUM > > | a whole table sweep is forced if the table hasn't been fully > | scanned for vacuum_freeze_table_age minus vacuum_freeze_min_age > | transactions. > > So reducing vacuum_freeze_min_age not only helps minimize the > writes that are needed when autovacuum needs to scan the entire > heap, but also decreases the frequency of those full-table scans. But it increases the amount of pages that are written out multiple times because they contain tuples of different ages, in contrast to increasing vacuum_freeze_table_age which doesn't have that problem. In combination with full_page_writes that makes a noticeable different in total write volume. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
