Re: fallocate mode flag for "unshare blocks"?

On Thu, Mar 31, 2016 at 07:18:55AM -0400, Austin S. Hemmelgarn wrote:

On 2016-03-30 20:32, Liu Bo wrote:

quoted

On Wed, Mar 30, 2016 at 11:27:55AM -0700, Darrick J. Wong wrote:

quoted

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:

quoted

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.

Even if glibc adds a check like that and emulates fallocate by writing
zero to real blocks, btrfs still does cow and requests to allocate space
for new writes, so it's not only performance, but also getting ENOSPC in
extremely case though.

Thanks,

-liubo