On 2014-04-03 15:02:27 +0300, Heikki Linnakangas wrote: > On 04/03/2014 02:41 PM, Andres Freund wrote: > >On 2014-04-03 13:38:29 +0300, Heikki Linnakangas wrote: > >>On 04/01/2014 08:58 PM, Andres Freund wrote: > >>>On 2014-04-01 12:56:04 -0500, Jim Nasby wrote: > >>>>On 3/4/14, 8:50 AM, Andres Freund wrote: > >>>>>Can't that be solved by just creating the permanent relation in a new > >>>>>relfilenode? That's equivalent to a rewrite, yes, but we need to do that > >>>>>for anything but wal_level=minimal anyway. > >>>> > >>>>Maybe I'm missing something, but doesn't this actually involve writing > >>>>the data twice? Once into WAL and again into the relation itself? > >>> > >>>Yes. But as I said, that's unavoidable for anything but > >>>wal_level=minimal. > >> > >>Ideally, you would *only* write the data to WAL, when you do ALTER TABLE ... > >>SET LOGGED. There's no fundamental reason you need to rewrite the > >>heap, too. > > > >As another point: What's the advantage of that? The amount of writes > >will be the same, no? It doesn't seem to be all that interesting that > >a second filenode exists temporarily? > > Surely it's cheaper to read the whole relation and copy it to just WAL, than > to read the whole relation and write it both the WAL and another file.
I have to admit I was thinking of the WAL replay case ;). But we'll actually have to write all dirty s_b, change the persistency tags and such anyway because there's no LSN interlock with checkpoints. That seems pretty ugly as well, and once again, avoidable by a rewrite. 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
