On 2013-02-11 11:17:16 -0800, Josh Berkus wrote: > On 02/11/2013 06:33 AM, Boszormenyi Zoltan wrote: > > 2013-02-11 15:25 keltezéssel, Andres Freund írta: > >> On 2013-02-11 15:21:13 +0100, Boszormenyi Zoltan wrote: > >>> 2013-01-24 18:02 keltezéssel, Tom Lane írta: > >>>> Andres Freund <and...@2ndquadrant.com> writes: > >>>>> On 2013-01-24 11:22:52 -0500, Tom Lane wrote: > >>>>>> Say again? Surely the temp file is being written by whichever > >>>>>> backend > >>>>>> is executing SET PERSISTENT, and there could be more than one. > >>>>> Sure, but the patch acquires SetPersistentLock exlusively beforehand > >>>>> which seems fine to me. > >>>> Why should we have such a lock? Seems like that will probably > >>>> introduce > >>>> as many problems as it fixes. Deadlock risk, blockages, etc. It is > >>>> not > >>>> necessary for atomicity, since rename() would be atomic already. > >>> There is a problem when running SET PERSISTENT for different GUCs > >>> in parallel. All happen to read the same original file, and only one > >>> setting ends up in the result if you rely only on the rename() being > >>> atomic. > >>> The LWLock provides the serialization for that problem. > >> Tom was voting for one-setting-per-file, in that case the problem > >> doesn't exist. > > > > I voted for the one-file approach and was arguing from the POV > > of the current implementation. > > I thought we discussed this ad naseum, and decided to go with the > one-single-file approach for the first round, since we already had an > implementation for that. I still think that's the right approach to take > with this patch; if it doesn't work out, we can go do > one-file-per-setting in 9.4.
Well, several people (at least Tom, I, and I think Zoltan as well) think that the one-file approach is considerably more complex. Check http://www.postgresql.org/message-id/20130126162728.ga5...@awork2.anarazel.de and related messages for some of the problems. Most of which are unhandled in the current patch, i.e. currently you *will* loose changes made in concurrent sessions. Greetings, Andres Freund -- 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
