Re: Snapshot target and DAX-capable devices

Kani, Toshi
2018-12-12T19:49:51Z

On Wed, 2018-12-12 at 12:50 -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).
>
> 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.

Just to be clear, "reintroduce DAX support to dm-snapshot" is a bit
misleading because there wasn't such attempt before. My original
hack/change in dm-snapshot was to support non-DAX use-cases to work with
DAX-capable devices.

Thanks,
-Toshi