Re: cheaper snapshots

On Thu, 2011-07-28 at 10:45 -0400, Tom Lane wrote:
> Hannu Krosing <hannu(at)2ndQuadrant(dot)com> writes:
> > On Thu, 2011-07-28 at 10:23 -0400, Robert Haas wrote:
> >> I'm confused by this, because I don't think any of this can be done
> >> when we insert the commit record into the WAL stream.
>
> > The update to stored snapshot needs to happen at the moment when the WAL
> > record is considered to be "on stable storage", so the "current
> > snapshot" update presumably can be done by the same process which forces
> > it to stable storage, with the same contention pattern that applies to
> > writing WAL records, no ?
>
> No. There is no reason to tie this to fsyncing WAL. For purposes of
> other currently-running transactions, the commit can be considered to
> occur at the instant the commit record is inserted into WAL buffers.
> If we crash before that makes it to disk, no problem, because nothing
> those other transactions did will have made it to disk either.

Agreed. Actually figured it out right after pushing send :)

> The
> advantage of defining it that way is you don't have weirdly different
> behaviors for sync and async transactions.

My main point was, that we already do synchronization when writing wal,
why not piggyback on this to also update latest snapshot .

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Hannu Krosing
PostgreSQL (Infinite) Scalability and Performance Consultant
PG Admin Book: http://www.2ndQuadrant.com/books/