linux-kernel - Re: [dm-devel] [RFC] dm-bow working prototype

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Message-ID: <20181025000123.GB22722@agk-dp.fab.redhat.com>
Date:   Thu, 25 Oct 2018 01:01:23 +0100
From:   Alasdair G Kergon <agk@...hat.com>
To:     Paul Lawrence <paullawrence@...gle.com>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, linux-doc@...r.kernel.org,
        kernel-team@...roid.com, Jonathan Corbet <corbet@....net>,
        linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        dm-devel@...hat.com, Shaohua Li <shli@...nel.org>
Subject: Re: [dm-devel] [RFC] dm-bow working prototype

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