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Re: [Qemu-devel] [PATCH v7 3/6] qcow2: Reduce REFT_OFFSET_MASK

From: Daniel P . Berrangé
Subject: Re: [Qemu-devel] [PATCH v7 3/6] qcow2: Reduce REFT_OFFSET_MASK
Date: Fri, 29 Jun 2018 16:43:00 +0100
User-agent: Mutt/1.10.0 (2018-05-17)

On Fri, Jun 29, 2018 at 10:22:22AM -0500, Eric Blake wrote:
> On 06/29/2018 03:44 AM, Kevin Wolf wrote:
> > Am 28.06.2018 um 21:07 hat Eric Blake geschrieben:
> > > Match our code to the spec change in the previous patch - there's
> > > no reason for the refcount table to allow larger offsets than the
> > > L1/L2 tables.
> > 
> > What about internal snapshots? And anyway, because of the metadata
> > overhead, the physical image size of a fully allocated image is always
> > going to be at least minimally larger than the virtual disk size.
> > 
> > I'm not necessarily opposed to making the change if there is a good
> > reason to make it, but I don't see a real need for it and the
> > justification used here and also in the previous patch is incorrect.
> The fact that ext4 cannot hold an image this large is already an indication
> that setting this limit on the refcount table is NOT going to bite real
> users.

NB, RHEL-7 defaults to xfs and this supports file sizes way larger
than ext4 does, so not sure we should consider ext4 as representative
of real world limits anymore.

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