Re: [Qemu-devel] [PATCH 2/2] specs/qcow2: Clarify that compressed clusters have the COPIED bit reset

[Eric Blake <eblake@redhat.com>]

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On 04/10/2018 11:05 AM, Alberto Garcia wrote:
> Compressed clusters are not supposed to have the COPIED bit set, but
> this is not made explicit in the specs, so let's document it.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  docs/interop/qcow2.txt | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
> index feb711fb6a..8e1547ded2 100644
> --- a/docs/interop/qcow2.txt
> +++ b/docs/interop/qcow2.txt
> @@ -400,10 +400,10 @@ L2 table entry:
>                62:   0 for standard clusters
>                      1 for compressed clusters
>  
> -              63:   0 for a cluster that is unused or requires COW, 1 if its
> -                    refcount is exactly one. This information is only accurate
> -                    in L2 tables that are reachable from the active L1
> -                    table.
> +              63:   0 for clusters that are unused, compressed or require COW.
> +                    1 for standard clusters whose refcount is exactly one.
> +                    This information is only accurate in L2 tables
> +                    that are reachable from the active L1 table.

This matches what qemu outputs, so the question becomes whether it is
technically necessary to make this requirement mandatory for 3rd-party
implementations.  But I'm in favor of the tighter wording, as it gets
rather hairy to check whether exactly one compressed cluster is
occupying a host cluster, plus I don't want to think about what happens
if a compressed cluster with the bit set crosses a host cluster boundary
(does it mean that compressed cluster is the only [remaining] source of
data for BOTH host clusters at once, where both the head of the first
host cluster and tail of the second host cluster is unused?)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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