Re: Snapshot target and DAX-capable devices

[Mike Snitzer <snitzer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, Dec 12 2018 at 11:12am -0500,
Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org> wrote:

> Does it really make sense to enhance dm-snapshot?  I thought all serious
> users of snapshots had moved on to dm-thinp?

There are cases where dm-snapshot is still useful for people.  But those
are very niche users.  I'm not opposed to others proposing enhancements
for dm-snapshot in general but it is definitely not a priority (Google's
dm-bow is an example of a case where dm-snapshot may get extended to
fulfill google's needs).

But for this specific DAX case, I can only assume efforts to prop up
dm-snapshot like this are born out of legacy use-cases.  The reality is
getting DAX to work with dm-snapshot is pretty involved (due to mmap,
etc).  This thread got into a lot of the details:
https://www.redhat.com/archives/dm-devel/2018-August/msg00211.html

So any new attempt to reintroduce DAX support to dm-snapshot (or any
more complex DM target) will have a very high bar.  One that requires
much more extensive mm and block (interlock) prep work to make DAX
safe.

Mike