On Thu, Oct 24, 2013 at 11:14:14PM +0300, Heikki Linnakangas wrote: > On 24.10.2013 23:07, Josh Berkus wrote: > >On 10/24/2013 11:12 AM, Heikki Linnakangas wrote: > >>On 24.10.2013 20:39, Josh Berkus wrote: > >>>On 10/24/2013 04:15 AM, Pavan Deolasee wrote: > >>>>If we do what you are suggesting, it seems like a single line patch > >>>>to me. > >>>>In XLogSaveBufferForHint(), we probably need to look at this > >>>>additional GUC > >>>>to decide whether or not to backup the block. > >>> > >>>Wait, what? Why are we having an additional GUC? > >>> > >>>I'm opposed to the idea of having a GUC to enable failback. When would > >>>anyone using replication ever want to disable that? > >> > >>For example, if you're not replicating for high availability purposes, > >>but to keep a reporting standby up-to-date. > > > >What kind of overhead are we talking about here? You probably said, but > >I've had a mail client meltdown and lost a lot of my -hackers emails. > > 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 thought that was the down-side to WAL logging hint bits, not the WAL volume itself. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + Everyone has their own god. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
