On Wed, Dec 12 2018 at 4:15pm -0500, Theodore Y. Ts'o <tytso@xxxxxxx> wrote: > 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@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). > > 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". dm-snapshot is only ~60% performant for 1 snapshot. Try to do additional snapshots and performance crawls to a stop (though I haven't reassessed performance in a while). dm-snapshot has been in Linux since before 2005, I don't know of all the users of it -- maybe there are a ton of users who only take a single temporary snapshot and we're all oblivious. Definitely not seeing many bugs against it (but it has been around forever). I do know that there are relatively few people showing interest in it. But for 4.21 I did stage a couple useful performance fixes: https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.21&id=61d594bb7e1cf86dca49cbc9524eb80169d9fca6 https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.21&id=d1f7898c7a1b24aa9ae670f9cc21b65e730827eb > 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. dm-thinp has the concept of an "external origin". Changes to origin LVM volume get copied out to the thin-pool (same copy cost as old dm-snapshot). But IIRC from that point on your LVM volume is a dm-thin device. Mike
