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Date: Fri, 10 Sep 2010 16:49:02 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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Subject: Re: [Qemu-devel] Re: [RFC] qed: Add QEMU Enhanced Disk format
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org

On 09/06/2010 05:45 AM, Anthony Liguori wrote:
>>
>> Before inventing yet another image format, you certainly have checked
>> the existing ones.
> 
> Obviously, yes.
> 
> Here are the issues:
> 
> cow.c: it's cow of an otherwise sparse file.  An important reason for 
> implementing a format is the ability to copy (or scp) an image without 
> special tools.
> 
> qcow2.c: the refcount, cow cluster, and compression make an 
> implementation seeking integrity and performance challenging.
> 
> vmdk.c: we feel it's important for qemu to have a block format with a 
> gpl friendly specification that we have a say in
> 
> vhd/vpc.c: same as vmdk with the addition that the OSP is known to not 
> be gpl friendly
> 
> vdi.c: uses a bitmap instead of a two level table.  An advantage of a 
> two level table is that it allows image resize without much fuss.
> 
> qcow.c: it lacks extensibility and compression means that there's no 
> guarantee that blocks are a fixed size.  This makes it very difficult to 
> implement a high performance block format without having two separate 
> code paths.
> 

Okay... how about, say, Linux LVM2 format?

	-hpa