Re: [PATCH v5 8/9] vfs: Add vfs_copy_file_range() support for pagecache copies

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

On Fri, Oct 16, 2015 at 01:02:23PM +0100, P??draig Brady wrote:
> Right. reflinking is transparent to the user, though its consequences are not.
> Consequences being the possible extra latency or ENOSPC on CoW.

You can get all these consequences without doing the file system reflink
by using a COW file system, any dedup scheme or thinly provisioned or
COW storage devices.

> Therefore reflinking should be an explicit action/flag IMHO.

This still does not make any sense, as it only prevents one of many
ways a file could do COW operations underneath.  If you don't want
ENOSPC use fallocate, or the proposed COPY_FALLOC flag.  If you want
care about latency you need to carefull benchmark your setup but in
general falloc / COPY_FALLOC might be a good starting point.  But for
99% of the copies a reflink is exactly the right thing to do.