Date: Sun, 18 Oct 2015 14:40:15 +0000
From: Hugo Mills <hugo@carfax.org.uk>
To: Rich Freeman <r-btrfs@thefreemanclan.net>
Cc: Xavier Gnata <xavier.gnata@gmail.com>,
        "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Subject: Re: btrfs autodefrag?
Message-ID: <20151018144015.GV25907@carfax.org.uk>
References: <56227910.7000208@gmail.com>
 <CAGfcS_kBeqT6bQKJNegjkcFNcEyqNF2ETg_KY64Y-7-2mc+9-g@mail.gmail.com>
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On Sun, Oct 18, 2015 at 10:24:39AM -0400, Rich Freeman wrote:
> On Sat, Oct 17, 2015 at 12:36 PM, Xavier Gnata <xavier.gnata@gmail.com> wrote:
> > 2) Disabling copy-on-write for just the VM image directory.
> 
> Unless this has changed, doing this will also disable checksumming.  I
> don't see any reason why it has to, but it does.  So, I avoid using
> this at all costs.

   It has to be disabled because if you enable it, there's a race
condition: since you're overwriting existing data (rather than CoWing
it), you can't update the checksums atomically. So, in the interests
of consistency, checksums are disabled.

   Hugo.

-- 
Hugo Mills             | Nothing wrong with being written in Perl... Some of
hugo@... carfax.org.uk | my best friends are written in Perl.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |                                                  dark

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