Re: [Qemu-devel] [PATCH 0/7] qed: Add QEMU Enhanced Disk format

[Stefan Hajnoczi <stefanha@gmail.com>]

On Mon, Sep 27, 2010 at 11:13 AM, Avi Kivity <avi@redhat.com> wrote:
>  On 09/23/2010 05:41 PM, Stefan Hajnoczi wrote:
>>
>> QEMU Enhanced Disk format is a disk image format that forgoes features
>> found in qcow2 in favor of better levels of performance and data
>> integrity.  Due to its simpler on-disk layout, it is possible to safely
>> perform metadata updates more efficiently.
>>
>> Installations, suspend-to-disk, and other allocation-heavy I/O workloads
>> will see increased performance due to fewer I/Os and syncs.  Workloads
>> that do not cause new clusters to be allocated will perform similar to
>> raw images due to in-memory metadata caching.
>>
>> The format supports sparse disk images.  It does not rely on the host
>> filesystem holes feature, making it a good choice for sparse disk images
>> that need to be transferred over channels where holes are not supported.
>>
>> Backing files are supported so only deltas against a base image can be
>> stored.
>>
>> The file format is extensible so that additional features can be added
>> later with graceful compatibility handling.
>>
>> Internal snapshots are not supported.  This eliminates the need for
>> additional metadata to track copy-on-write clusters.
>>
>> Compression and encryption are not supported.  They add complexity and can
>> be
>> implemented at other layers in the stack (i.e. inside the guest or on the
>> host).  Encryption has been identified as a potential future extension and
>> the
>> file format allows for this.
>>
>
> IMO the file format should be part of this patchset.  Wikis are nice but
> they aren't a good place for authoritative documentation.

Good idea.  Will add the specification in v2.

Stefan