Re: pgsql-server/src backend/storage/buffer/bufmgr ...

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Tom Lane wrote:
> >> As I've said before, I think we need to find a way to stop using sync()
> >> altogether --- we have to move to fsync or O_SYNC and variants.  sync
> >> has simply got the wrong API.
>
> > If sync failes (kernel to disk write failes) we have a hardware failure,
> > and we don't pretend to recover from that,
>
> Not necessarily --- it could be out-of-disk-space, on at least some
> filesystems.  More to the point, the important thing is not to commit a
> checkpoint record to WAL indicating that everything is good, when
> everything is not good.  As long as we don't checkpoint we have some
> hope of recovering automatically via WAL replay.
>
> > One idea I floated around was to
> > open/write/fsync/close a temporary file after sync in the hope that it
> > would happen after the sync completes because the fsync would be at the
> > end of the disk flush queue.
>
> "In the hope"?  We already have a guess-and-hope approach to this, and
> it will never be any better as long as we use sync(), because sync() is
> fundamentally the wrong operation.  It doesn't tell you when the I/O is
> done, and it doesn't tell you whether the I/O was done successfully, and
> there is no possibility of working around that fundamental lack of
> information except to stop using it.

I guess my major problem with moving away from sync is similar to the
reason we don't do raw devices --- sync is best done in the kernel and
disk driver that knows more about how to do it efficiently.  I haven't
see any non-sync solution with performance similar to sync().  However,
we are going to have to write one for win32, so we can test things once
we are done and then decide.

I think the win32 solution will be to record modified files in a central
location, and have the checkpoint open/fsync(_commit), perhaps it all
happening at the same time in different threads so it isn't serialized.

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073