Re: fallocate mode flag for "unshare blocks"?

["Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>]

On 2016-03-30 20:32, Liu Bo wrote:
> On Wed, Mar 30, 2016 at 11:27:55AM -0700, Darrick J. Wong wrote:
> > Hi all,
> >
> > Christoph and I have been working on adding reflink and CoW support to
> > XFS recently.  Since the purpose of (mode 0) fallocate is to make sure
> > that future file writes cannot ENOSPC, I extended the XFS fallocate
> > handler to unshare any shared blocks via the copy on write mechanism I
> > built for it.  However, Christoph shared the following concerns with
> > me about that interpretation:
> >
> > > I know that I suggested unsharing blocks on fallocate, but it turns out
> > > this is causing problems.  Applications expect falloc to be a fast
> > > metadata operation, and copying a potentially large number of blocks
> > > is against that expextation.  This is especially bad for the NFS
> > > server, which should not be blocked for a long time in a synchronous
> > > operation.
> > >
> > > I think we'll have to remove the unshare and just fail the fallocate
> > > for a reflinked region for now.  I still think it makes sense to expose
> > > an unshare operation, and we probably should make that another
> > > fallocate mode.
> I'm expecting fallocate to be fast, too.
>
> Well, btrfs fallocate doesn't allocate space if it's a shared one
> because it thinks the space is already allocated.  So a later overwrite
> over this shared extent may hit enospc errors.
And this _really_ should get fixed, otherwise glibc will add a check for 
running posix_fallocate against BTRFS and force emulation, and people 
_will_ complain about performance.