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Date:   Thu, 25 Oct 2018 01:01:23 +0100
From:   Alasdair G Kergon <agk@redhat.com>
To:     Paul Lawrence <paullawrence@google.com>
Cc:     Mikulas Patocka <mpatocka@redhat.com>,
        Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@redhat.com>, linux-doc@vger.kernel.org,
        kernel-team@android.com, Jonathan Corbet <corbet@lwn.net>,
        linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org,
        dm-devel@redhat.com, Shaohua Li <shli@kernel.org>
Subject: Re: [dm-devel] [RFC] dm-bow working prototype
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On Wed, Oct 24, 2018 at 03:24:29PM -0400, Mikulas Patocka wrote:
> What about allocating a big file, using the FIEMAP ioctl to find the 

For reference, dmfilemapd in the lvm2 tree (in daemons/) uses FIEMAP
(in libdm/libdm-stats.c) for monitoring I/O by file.

> If you decide that rollback is no longer needed, you just unload the 
> snapshot target and delete the big file. If you decide that you want to 
> rollback, you can use the snapshot merge functionality (or you can write a 
> userspace utility that does offline merge).
 
There's some old code from Mark McLoughlin for userspace snapshot merging here:
  https://people.gnome.org/~markmc/code/merge-dm-snapshot.c

Alasdair