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

[Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

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

> 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 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.
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