From: Mike Snitzer <snitzer(a)redhat.com>
Sent: Thursday, December 13, 2018 6:43 AM
On Wed, Dec 12 2018 at 4:15pm -0500,
Theodore Y. Ts'o <tytso(a)mit.edu> 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(a)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".
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
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
[RFC PATCH v2 3/3] dm: expand valid types for dm-ioctl