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Re: [Qemu-block] [Qemu-devel] QCow2 compression
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From: |
Eric Blake |
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Subject: |
Re: [Qemu-block] [Qemu-devel] QCow2 compression |
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Date: |
Mon, 29 Feb 2016 08:54:21 -0700 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 |
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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