[Sheepdog] [PATCH] sheep: unify create_and_write logic for cow and regular object

[Christoph Hellwig]

On Tue, Nov 15, 2011 at 06:18:21AM -0500, tao.peng at emc.com wrote:
> > On Tue, Nov 15, 2011 at 04:18:52PM +0800, Liu Yuan wrote:
> > > Umm, seems useless, I just squeeze it to the code path. any noticeable
> > > performance penalty?
> > 
> > With the current O_SYNC I/O model it at least means an additional log
> > force, which on disks will be very noticeable.  So unless you really
> > see a difference in I/O patterns don't do it.
> Just out of curiosity, won't fallocate() be a better hint to underlying file system which can then create a better on disk layout? With O_SYNC IO it forces allocation on each write...

Currently sheepdog does not support a writeback cache model yet, so we
have to use O_SYNC everywhere.   Even without that it will not provide
a benefit for any reaosnable filesystem.

For O_DIRECT I/O (which I would recommend for sheepdog nodes) the
filesystem always sees the whole I/O and thus can trivially allocate
one extent for the object.  For buffered I/O the filesystem writepage
method sees that the whole object is in delayed allocation state and
can convert the whole extent to a real extent (at least that's what
XFS does).