Re: [Qemu-devel] [RFC V3 01/24] qcow2: Add deduplication to the qcow2 specification.

[Eric Blake <eblake@redhat.com>]

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On 11/26/2012 06:05 AM, Benoît Canet wrote:
> Signed-off-by: Benoit Canet <benoit@irqsave.net>
> ---
>  docs/specs/qcow2.txt |   33 ++++++++++++++++++++++++++++++++-
>  1 file changed, 32 insertions(+), 1 deletion(-)
> 

In addition to Stefan's comments,

> @@ -159,6 +163,33 @@ the header extension data. Each entry look like this:
>                      terminated if it has full length)
>  
>  
> +== Deduplication ==
> +
> +The deduplication extension contains the offset and size of the deduplication
> +table.
> +
> +    Byte   0 - 7:   Offset
> +
> +          8 - 11:   Size
> +
> +== Deduplication table ==
> +
> +The deduplication table contains 64 bits offsets to the level 2 deduplication

s/64 bits/64-bit/

> +table clusters.
> +Each entry of these clusters contains a 32 bytes SHA256 hash followed by the

s/32 bytes/32-byte/

> +64 bits logical offset of the first encountered block having this hash.

s/64 bits/64-bit/

> +
> +Entries in the deduplication table are orderered by physical cluster index.

s/orderered/ordered/

> +
> +The number of entries in an l2 deduplication table cluster is :
> +l2_dedup_cluster_entries = cluster_size / (32 + 8)

32+8 is not a power of two; what happens to the tail bytes at the end of
a cluster of entries?  If you define them to be 0 now, you can use them
for possible extensions later.

> +
> +The index in the level 1 deduplication table is :
> +l1_dedup_index = physical_cluster_index / l2_dedup_cluster_entries
> +
> +The index in the level 2 deduplication table is:
> +l2_dedup_index = physical_cluster_index % l2_dedup_cluster_entries
> +
>  == Host cluster management ==
>  
>  qcow2 manages the allocation of host clusters by maintaining a reference count
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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