From: Roman Mamedov <rm@romanrm.net>
To: Duncan <1i5t5.duncan@cox.net>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: [virt-devel] btrfs NOCOW for VM disk images
Date: Sat, 23 Nov 2013 04:00:28 +0600 [thread overview]
Message-ID: <20131123040028.71a8ba4e@natsu> (raw)
In-Reply-To: <pan$25aa5$1906a260$3a199c05$d941ebdc@cox.net>
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On Fri, 22 Nov 2013 21:26:16 +0000 (UTC)
Duncan <1i5t5.duncan@cox.net> wrote:
> > During testing I found that the NOCOW flag prevents file cloning from
> > working. cp --reflink fails with EINVAL when the source file has the
> > NOCOW flag set.
>
> That would be expected, since disabling COW means the file will be
> updated in-place, and if reflink-copying was allowed, changing the one
> view in-place would by definition change the other view of the same file,
> since it /is/ the same file data.
However snapshotting a subvolume which has NOCOW files *is* allowed.
I'm told data is then COW'ed only once, and only the areas that are changed
after the snapshot has been made (or something along those lines). So since
snapshotting+NOCOW can be combined and everything works automagically as
expected, maybe reflink could be made to work as well?
--
With respect,
Roman
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next prev parent reply other threads:[~2013-11-22 22:00 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20131122142051.GA32192@stefanha-thinkpad.redhat.com>
2013-11-22 16:17 ` Fwd: [virt-devel] btrfs NOCOW for VM disk images John Dulaney
2013-11-22 21:26 ` Duncan
2013-11-22 22:00 ` Roman Mamedov [this message]
2013-11-23 1:21 ` David Sterba
2013-11-22 22:12 ` Chris Murphy
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