Re: [Qemu-devel] QCow2 compression

[Eric Blake <eblake@redhat.com>]

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On 02/29/2016 07:59 AM, Eric Blake wrote:

>> an L2 entry value of 4A C0 00 00 00 3D 97 50.
> 
> So with default 64k clusters, x = 62 - (16 - 8) = 54.  Bits 0-54 are the
> host cluster offset, or 0x003d9750, but that is in terms of host
> sectors.  The comment in block/qcow2.c is telling, and perhaps we should
> improve the qcow2 spec to make it obvious:
> 
>   - Size of compressed clusters is stored in sectors to reduce bit usage
>     in the cluster offsets.
> 
> Thus, in your image, the guest compressed data starts at sector
> 0x003d9750, or host file offset 0x7b2ea000.  This value is NOT aligned
> to a cluster, but IS aligned to a sector (since a sector is the smallest
> unit we write to), and makes more sense than something ending in 0x50
> (which is not sector aligned).

Disclaimer - I did not test things, so I may be misreading the spec myself.

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


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