On Wed, 2018-12-12 at 12:50 -0500, Mike Snitzer wrote: > On Wed, Dec 12 2018 at 11:12am -0500, > Christoph Hellwig <hch@xxxxxxxxxxxxx> 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
