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Date: Wed, 14 Aug 2013 16:58:55 +0100
From: "Richard W.M. Jones" <rjones@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] Introduce cache images for the QCOW2 format
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To: Alex Bligh <alex@alex.org.uk>
Cc: Kevin Wolf <kwolf@redhat.com>, Kaveh Razavi <kaveh@cs.vu.nl>, famz@redhat.com, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>


On Wed, Aug 14, 2013 at 01:03:48PM +0100, Alex Bligh wrote:
> 
> On 14 Aug 2013, at 12:52, Fam Zheng wrote:
> 
> > Yes, this one sounds good to have. VMDK and VHDX have this kind of
> > backing file status validation.
> 
> ... though I'd prefer something safer than looking at mtime, for
> instance a sequence number that is incremented prior to any
> bdrv_close if a write has been done since bdrv_open.

Yes, please not mtime.  User-mode Linux COW files use mtime to check
this, and it causes no end of problems (eg. if the backing file is
copied from one place to another without using the magic incantations
to preserve file times).

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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