Bruce Momjian escribió: > On Thu, Oct 24, 2013 at 11:14:14PM +0300, Heikki Linnakangas wrote: > > On 24.10.2013 23:07, Josh Berkus wrote:
> > >What kind of overhead are we talking about here? > > > > One extra WAL record whenever a hint bit is set on a page, for the > > first time after a checkpoint. In other words, a WAL record needs to > > be written in the same circumstances as with page checksums, but the > > WAL records are much smaller as they don't need to contain a full > > page image, just the block number of the changed block. > > > > Or maybe we'll write the full page image after all, like with page > > checksums, just without calculating the checksums. It might be > > tricky to skip the full-page image, because then a subsequent change > > of the page (which isn't just a hint-bit update) needs to somehow > > know it needs to take a full page image even though a WAL record for > > it was already written. > > Sorry to be replying late to this, but while I am not worried about the > additional WAL volume, does this change require the transaction to now > wait for a WAL sync to disk before continuing? I don't think so. There's extra WAL written, but there's no flush-and-wait until end of transaction (as has always been). > I thought that was the down-side to WAL logging hint bits, not the WAL > volume itself. I don't think this is true either. -- Álvaro Herrera 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
