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Re: [Qemu-block] [PATCH v3 2/4] qcow2: Document some maximum size constr

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH v3 2/4] qcow2: Document some maximum size constraints
Date: Tue, 27 Feb 2018 12:47:02 +0100
User-agent: Mutt/1.9.1 (2017-09-22)

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 <address@hidden>

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.


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