On 2013-05-12 19:41:26 -0500, Jon Nelson wrote:
> On Sun, May 12, 2013 at 3:46 PM, Jim Nasby <jim@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
