On 2015-10-16 08:24, Christoph Hellwig wrote:

On Fri, Oct 16, 2015 at 01:02:23PM +0100, P??draig Brady wrote:

quoted

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.

quoted

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.

There is at least one reason other than avoiding ENOSPC and minimizing 
latency that people may want to avoid reflinking things: They actually 
_want_ multiple physically independent copies of the same file on the 
disk.  Usually people do go about this wrong (some people I know don't 
understand that having multiple copies of a file on the same filesystem 
provides no greater safety than one copy), but that doesn't mean that 
this isn't a perfectly valid use case for copying a file.