fallocate mode flag for "unshare blocks"?

[Austin S. Hemmelgarn]

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.