On Tue, 2008-10-28 at 10:59 +0000, Gregory Stark wrote: > Simon Riggs <[EMAIL PROTECTED]> writes: > > > I don't see a reason why we would issue 2 WAL records per block for a > > VACUUM, nor why we would prune and remove in two steps, dirtying the > > block each time. Seems like we could write approximately half the amount > > of data that we do. > > > > Surely we can come up with a better plan than that one? > > This sounds like the same issue Pavan identified and proposed solving by > rotating the three passes so that we do step 3 at the beginning of the next > vacuum run, so it can be done in the same pass as step 1. > > To do that he proposed we do: > > 1. scan heap doing two things: a) remove any marked tuples if they were marked > by a previous vacuum which committed and b) prune and mark any tuples we > find are deletable for a future vacuum to remove. > > 2. scan indexes and remove the tuples we marked in 1b.
It's fairly hard to remove the second heap pass completely. I think what I am suggesting is two heap passes, but writing WAL and dirtying blocks on only one of the passes. The biggest I/O cost comes from the writes, not the reads, ISTM. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
