On 2014-05-15 16:13:49 -0400, Bruce Momjian wrote: > On Thu, May 15, 2014 at 10:06:32PM +0200, Andres Freund wrote: > > > If the larger clog size is a show-stopper (and I'm not sure I have an > > > intelligent opinion on that just yet), one way to get around the > > > problem would be to summarize CLOG entries after-the-fact. Once an > > > XID precedes the xmin of every snapshot, we don't need to know the > > > commit LSN any more. So we could read the old pg_clog files and write > > > new summary files. Since we don't need to care about subcommitted > > > transactions either, we could get by with just 1 bit per transaction, > > > 1 = committed, 0 = aborted. Once we've written and fsync'd the > > > summary files, we could throw away the original files. That might > > > leave us with a smaller pg_clog than what we have today. > > > > I think the easiest way for now would be to have pg_clog with the same > > format as today and a rangewise much smaller pg_csn storing the lsns > > that are needed. That'll leave us with pg_upgrade'ability without > > needing to rewrite pg_clog during the upgrade. > > Yes, I like the idea of storing the CSN separately. One reason the > 2-bit clog is so good is that we know we have atomic 1-byte writes on > all platforms.
I don't think we rely on that anywhere. And in fact we don't have the ability to do so for arbitrary bytes - lots of platforms can do that only on specifically aligned bytes. We rely on being able to atomically (as in either before/after no torn value) write/read TransactionIds, but that's it I think? > Can we assume atomic 64-bit writes? Not on 32bit platforms. 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