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Date: Wed, 12 Dec 2018 16:15:47 -0500
From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Mike Snitzer <snitzer@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>,
        Huaisheng Ye <yehs2007@zoho.com>, Jan Kara <jack@suse.cz>,
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Subject: Re: Snapshot target and DAX-capable devices
Message-ID: <20181212211547.GA24926@thunk.org>
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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".  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.

						- Ted