Re: [Qemu-devel] [PATCH 2/4] block: use fdatasync instead of fsync

[Jamie Lokier <jamie@shareable.org>]

Christoph Hellwig wrote:
> On Tue, Sep 01, 2009 at 06:59:46PM +0300, Blue Swirl wrote:
> > On Mon, Aug 31, 2009 at 11:17 PM, Christoph Hellwig<hch@lst.de> wrote:
> > >
> > > If we are flushing the caches for our image files we only care about the
> > > data (including the metadata required for accessing it) but not things
> > > like timestamp updates. ??So use fdatasync instead of fsync to implement
> > > the flush operations.
> > 
> > > - ?? ??fsync(s->fd);
> > > + ?? ??fdatasync(s->fd);
> > 
> > There is no fdatasync outside of Linux, not in OpenBSD and mingw32 at
> > least. But you could add a probe for it to configure.
> 
> Oh, okay.  Kinda sad that people still don't implement useful inteface
> that have been in Posix for a long time.

fdatasync is common among the big commercial unixes which were still
being actively developed a few years ago, but fsync is universal -
it's very old.

Neither OpenBSD nor FreeBSD have fdatasync.

mingw32 is a thin wrapper around Windows.  Windows as far as I can
tell doesn't have an equivalent of fdatasync, although it does have an
equivalent of O_DIRECT|O_SYNC (but documentation contradicts itself
regarding whether the combination flushes metadata on each write).

-- Jamie