Re: Snapshot target and DAX-capable devices

Theodore Y. Ts'o
2018-12-12T21:15:47Z

On Wed, Dec 12, 2018 at 12:50:47PM -0500, Mike Snitzer wrote:
> On Wed, Dec 12 2018 at 11:12am -0500,
> Christoph Hellwig <hch@infradead.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).

I would expect that dm-snapshot will be used quite a lot for
short-lived snapshots (that only live during a database backup or an
fsck run). I would hardly call that a "niche use case". One other
major advantage that dm-snapshot has is that you can take a snapshot
for any LVM volume. For dm-thinp you have migrate your storage to a
thinp pool, and that adds a fair amount of friction to users migrating
to dm-thinp.

- Ted