On Tuesday, June 19, 2012 09:55:30 PM Robert Haas wrote: > On Wed, Jun 13, 2012 at 7:28 AM, Andres Freund <and...@2ndquadrant.com> wrote: > > From: Andres Freund <and...@anarazel.de> > > > > The previous coding could miss xlog writeouts at several places. E.g. > > when wal was written out by the background writer or even after a commit > > if synchronous_commit=off. > > This could lead to delays in sending data to the standby of up to 7 > > seconds. > > > > To fix this move the responsibility of notification to the layer where > > the neccessary information is actually present. We take some care not to > > do the notification while we hold conteded locks like WALInsertLock or > > WalWriteLock locks. > > I am not convinced that it's a good idea to wake up every walsender > every time we do XLogInsert(). XLogInsert() is a super-hot code path, > and adding more overhead there doesn't seem warranted. We need to > replicate commit, commit prepared, etc. quickly, by why do we need to > worry about a short delay in replicating heap_insert/update/delete, > for example? They don't really matter until the commit arrives. 7 > seconds might be a bit long, but that could be fixed by decreasing the > polling interval for walsender to, say, a second. Its not woken up every XLogInsert call. Its only woken up if there was an actual disk write + fsync in there. Thats exactly the point of the patch. The wakeup rate is actually lower for synchronous_commit=on than before because then it unconditionally did a wakeup for every commit (and similar) and now only does that if something has been written + fsynced.
> Parenthetically, I find it difficult to extract inline patches. No > matter whether I try to use it using Gmail + show original or the web > site, something always seems to get garbled. Will use git send-mail --attach next time... Btw, git am should be able to extract the patches for you. Andres -- Andres Freund 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
