On 2013-05-12 19:41:26 -0500, Jon Nelson wrote: > On Sun, May 12, 2013 at 3:46 PM, Jim Nasby <j...@nasby.net> wrote: > > On 5/10/13 1:06 PM, Jeff Janes wrote: > >> > >> Of course the paranoid DBA could turn off restart_after_crash and do a > >> manual investigation on every crash, but in that case the database would > >> refuse to restart even in the case where it perfectly clear that all the > >> following WAL belongs to the recycled file and not the current file. > > > > > > Perhaps we should also allow for zeroing out WAL files before reuse (or just > > disable reuse). I know there's a performance hit there, but the reuse idea > > happened before we had bgWriter. Theoretically the overhead creating a new > > file would always fall to bgWriter and therefore not be a big deal. > > For filesystems like btrfs, re-using a WAL file is suboptimal to > simply creating a new one and removing the old one when it's no longer > required. Using fallocate (or posix_fallocate) (I have a patch for > that!) to create a new one is - by my tests - 28 times faster than the > currently-used method.
I don't think the comparison between just fallocate()ing and what we currently do is fair. fallocate() doesn't guarantee that the file is the same size after a crash, so you would still need an fsync() or we couldn't use fdatasync() anymore. And I'd guess the benefits aren't all that big anymore in that case? That said, using posix_fallocate seems like a good idea in lots of places inside pg, its just not all that easy to do in some of the places. 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
