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Date: Tue, 27 Feb 2018 12:47:02 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v3 2/4] qcow2: Document some maximum size
 constraints
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To: Eric Blake <eblake@redhat.com>
Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, berto@igalia.com, Max Reitz <mreitz@redhat.com>

Am 22.02.2018 um 16:59 hat Eric Blake geschrieben:
> Although off_t permits up to 63 bits (8EB) of file offsets, in
> practice, we're going to hit other limits first.  Document some
> of those limits in the qcow2 spec, and how choice of cluster size
> can influence some of the limits.
> 
> While at it, notice that since we cannot map any virtual cluster
> to any address higher than 64 PB (56 bits) (due to the L1/L2 field
> encoding), it makes little sense to require the refcount table to
> access host offsets beyond that point.  Mark the upper bits of
> the refcount table entries as reserved, with no ill effects, since
> it is unlikely that there are any existing images larger than 64PB
> in the first place, and thus all existing images already have those
> bits as 0.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

I think it would be good to mention the exact reason for the 56 bits in
the spec. Even this commit message is rather vague ('L1/L2 field
encoding'), so if at some point someone wonders, if we couldn't simply
extend the allowed range, they won't easily see that it's related to
compressed clusters.

Kevin