On 4/20/15 1:48 PM, Bruce Momjian wrote:
On Mon, Apr 20, 2015 at 04:45:34PM +0900, Sawada Masahiko wrote:
Attached WIP patch adds Frozen Map which enables us to avoid whole
table vacuuming even when full scan is required: preventing XID
wraparound failures.
Frozen Map is a bitmap with one bit per heap page, and quite similar
to Visibility Map. A set bit means that all tuples on heap page are
completely frozen, therefore we don't need to do vacuum freeze that
page.
A bit is set when vacuum(or autovacuum) figures out that all tuples on
corresponding heap page are completely frozen, and a bit is cleared
when INSERT and UPDATE(only new heap page) are executed.
So, this patch avoids reading the all-frozen pages if it has not been
modified since the last VACUUM FREEZE? Since it is already frozen, the
running VACUUM FREEZE will not modify the page or generate WAL, so is it
really worth maintaining a new per-page bitmap just to avoid the
sequential scan of tables every 200MB transactions? I would like to see
us reduce the need for VACUUM FREEZE, rather than go in this direction.
How would you propose we do that?
I also think there's better ways we could handle *all* our cleanup work.
Tuples have a definite lifespan, and there's potentially a lot of
efficiency to be gained if we could track tuples through their stages of
life... but I don't see any easy ways to do that.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com
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