Re: Snapshot target and DAX-capable devices

Mike Snitzer
2018-12-18T19:49:40Z

On Fri, Dec 14 2018 at 3:24am -0500,
Huaisheng HS1 Ye <yehs1@lenovo.com> wrote:

> From: Mike Snitzer <snitzer@redhat.com>
> Sent: Thursday, December 13, 2018 6:43 AM
> > On Wed, Dec 12 2018 at 4:15pm -0500,
> > Theodore Y. Ts'o <tytso@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@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
> > 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

No I'm not taking these patches. They are hacks that allow DM targets
to do things that aren't supportable (yet).