On Tue, 2008-10-28 at 08:49 +0000, Simon Riggs wrote:
> Looking at a VACUUM's WAL records makes me think twice about the way we
> issue a VACUUM.
>
> 1. First we scan the heap, issuing a HEAP2 clean record for every block
> that needs cleaning.
IIRC the first heap pass just collects info and does nothing else.
Is this just an empty/do-nothing WAL record ?
> 2. Then we scan the index, issuing WAL records as appropriate.
>
> 3. Then we rescan the heap, issuing a HEAP2 clean record for every
> block.
>
> 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.
The first pass should just be collecting info and not dirtying anything.
Could it be side effect of setting some transaction visibility bits on
first visit ?
In that case It would be good, if we could disable doing that that for
vacuum.
> 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?
-------------------
Hannu Krosing
http://www.2ndQuadrant.com
PostgreSQL Scalability Training, Services and Support