Re: [PATCH v5 9/9] btrfs: btrfs_copy_file_range() only supports reflinks

On Wed, Oct 14, 2015 at 11:46:08AM -0700, Darrick J. Wong wrote:

On Tue, Oct 13, 2015 at 12:29:59AM -0700, Christoph Hellwig wrote:

quoted

On Mon, Oct 12, 2015 at 04:41:06PM -0700, Darrick J. Wong wrote:

quoted

One of the patches in last week's XFS reflink patchbomb adds FALLOC_FL_UNSHARE
flag; at the moment it _only_ forces copy-on-write of shared blocks, and it
leaves holes alone.

Yes, I've seen the implementation. 

quoted

Obviously we haven't yet figured out what are peoples' preferences in terms of
"fill the holes and unshare the shared" vs. "only unshare the shared" vs. "only
fill the holes".  It isn't that hard to add a FALLOC_FL_UNSHARE_FILL_HOLES flag
that fills the holes while unsharing is going on.

Personally I suspect that the most interest is in filling holes and unsharing,
because they don't want to pay for allocation at a critical stage for anywhere
in the file.  But I could be wrong, so allowing both goals to be expressed via
mode allows flexibility.

Exactly.  And a normal falloc should do just that - fill holes and
ensure that we don't need to COW already allocated locks.  So I don't
think we need a new fallocate interface for that.

The documentation for fallocate ought to be updated to include that as part of
guaranteeing that subsequent writes to the range won't fail due to ENOSPC,
shared blocks will be unshared.

Incidentally, btrfs leaves shared blocks alone.  OTOH, given that it's totally
COW it probably doesn't make sense to unshare blocks anyway... but maybe I
also don't want to dive into btrfs f-allocation behavior at this time. :)

Ok, so I'll rework the XFS funshare code into something that hangs off the
regular fallocate call, and get rid of the explicit 'funshare' bits.

Makes sense given we have the FALLOC_FL_ZERO_RANGE operation which
returns a zero to and preallocates all the holes in the range. I
would expect this operation on shared blocks to unshare blocks,
too...

quoted

The question is if we
want a copy interface that gives you the same semantics as if you also
called an fallocate on the destination range.  For that case we'd
usually want to avoid doing the clone and instead do a in-kernel or
hardware assisted copy and then fill the holes with unwritten extents.

If hole filling was required, then I'd do the operation the other
way around - prealloc the entire range, then do hardware assisted
copy of each separate data range in the source file with unwritten
extent conversion on offload completion...

Probably; I can easily imagine people wanting to fill the holes and also
not wanting them filled.

*nod*.

Cheers,

Dave.
-- 
Dave Chinner
david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org
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