From: Mike Snitzer <snitzer@xxxxxxxxxx> Sent: Thursday, December 13, 2018 6:43 AM > 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 Hi Mike, Could these two patches be applied to current code of LVM? Although there is a difficult problem as mmap for dm-snapshot with DAX-capable, the two patches can be used for other complex DM targets when trying to implement DAX. [RFC PATCH v2 2/3] dm: expand hc_map in mapped_device for lack of map https://lkml.org/lkml/2018/11/21/273 [RFC PATCH v2 3/3] dm: expand valid types for dm-ioctl https://lkml.org/lkml/2018/11/21/276 Cheers, Huaisheng Ye
