On 2016-03-31 07:18, Austin S. Hemmelgarn wrote:
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.
Thinking a bit further about this, how hard would it be to add the
ability to have unwritten extents point somewhere else for reads? Then
when we get an fallocate call, we create the unwritten extents, and add
the metadata to make them read from the shared region. Then, when a
write gets issued to that extent, the parts that aren't being written in
that block get copied, the write happens, and then the link for that
block gets removed. This way, fallocate would still provide the correct
semantics, it would be relatively fast (still not quite as fast as it is
now, but it wouldn't be anywhere near as slow as copying the data), and
the cost of copying gets amortized across writes (we may not need to
copy everything, but we'll still copy less than we would for just
un-sharing the extent). This would of course need to be an incompat
feature, but I would personally say that's not as much of an issue, as
things are subtly broken in the common use-case right now (at this point
I'm just thinking BTRFS, as what Darrick suggested for XFS seems to be a
better solution there at least short term).
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