[Samba] Direct I/O

Christof Schmitt
Tue, 25 Jan 2022 00:50:01 -0000

On Mon, Jan 24, 2022 at 10:43:11AM -0800, Jeremy Allison via samba
wrote:
> On Mon, Jan 24, 2022 at 07:21:05PM +0100, Ralph Boehme wrote:
> > On 1/24/22 18:39, Jeremy Allison via samba wrote:
> > > No, we don't currently use O_DIRECT on Linux.
> > >
> > > It doesn't match to SMB2+ semantics.
> >
> > I think it does (SMB2_WRITEFLAG_WRITE_UNBUFFERED), but we don't
yet
> > implement it correctly. We check the flag and basically skip an
fsync()
> > done in some cases, but we don't pass it to the OS. I guess the
problem
> > is that in SMB the unbuffered is requested per *write* whereas we can
> > only request unbuffered IO per *handle* with O_DIRECT. But I may be
> > missing something.
>
> Oh, good catch. I missed that flag. However I think there's another
> missmatch. O_DIRECT has alignment restrictions on where the
> IO can be performed. I think it mandates 512 byte boundaries.

FWIW, preadv2() has RWF_DSYNC and RWF_SYNC which provide O_DSYNC and
O_SYNC semantics per write. Maybe one of those aligns more closely with
WRITE_UNBUFFERED.

Christof