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Date: Wed, 12 Dec 2018 12:50:47 -0500
From: Mike Snitzer <snitzer@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Huaisheng Ye <yehs2007@zoho.com>, Jan Kara <jack@suse.cz>,
        Dave Chinner <david@fromorbit.com>,
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Subject: Re: Snapshot target and DAX-capable devices
Message-ID: <20181212175047.GA24962@redhat.com>
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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).

But for this specific DAX case, I can only assume efforts to prop up
dm-snapshot like this are born out of legacy use-cases.  The reality is
getting DAX to work with dm-snapshot is pretty involved (due to mmap,
etc).  This thread got into a lot of the details:
https://www.redhat.com/archives/dm-devel/2018-August/msg00211.html

So any new attempt to reintroduce DAX support to dm-snapshot (or any
more complex DM target) will have a very high bar.  One that requires
much more extensive mm and block (interlock) prep work to make DAX
safe.

Mike