Re: [Qemu-devel] Re: [RFC] qed: Add QEMU Enhanced Disk format

[Kevin Wolf]

Am 06.09.2010 14:57, schrieb Anthony Liguori:
> On 09/06/2010 07:40 AM, Stefan Hajnoczi wrote:
>> On Mon, Sep 6, 2010 at 11:27 AM, Kevin Wolf<address@hidden>  wrote:
>>    
>>> Am 06.09.2010 12:04, schrieb Stefan Hajnoczi:
>>>      
>>>> 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).
>>>>
>>>> The format is currently functional with the following features missing:
>>>>   * Resizing the disk image.  The capability has been designed in but the
>>>>     code has not been written yet.
>>>>   * Resetting the image after backing file commit completes.
>>>>   * Changing the backing filename.
>>>>   * Consistency check (fsck).  This is simple due to the on-disk layout.
>>>>
>>>> Signed-off-by: Anthony Liguori<address@hidden>
>>>> Signed-off-by: Stefan Hajnoczi<address@hidden>
>>>>        
>>> Okay, so before I actually look at the patch longer than a couple of
>>> seconds let me just ask the obvious question...
>>>
>>> Before inventing yet another image format, you certainly have checked
>>> the existing ones. Except for not implementing compression and
>>> encryption this looks a lot like qcow1 to me. I see that you even
>>> retained the two-level cluster tables.
>>>
>>> So if we ignore the implementation for a moment and just compare the
>>> formats, what's the crucial difference between qcow1 and qed that I'm
>>> missing? And if it's not qcow1, why not improving our support for
>>> another existing format like VHD?
>>>      
>> Is this a subset of existing on-disk formats?  Yes.  The motivation is
>> to have an image format that performs well and is safe, with backing
>> image support.  Currently no image format in QEMU meets these
>> requirements.
>>
>> Perhaps it is appropriate to use an existing on-disk format.
> 
> If you implement a subset of functionality for an existing on-disk 
> format, I think you damage user's expectations.

I don't really buy that implementing compression/encryption wouldn't
have been possible if it was the only problem. Of course, if you don't
implement it, you can't use an on-disk format that supports them.

> If we claim to support qcow images, then given any old qcow image I have 
> laying around for 5 years ago, I should be able to run it without qemu 
> throwing an error.
> 
> There's some really ugly stuff in qcow.  Nothing is actually aligned.  
> This makes implementing things like O_DIRECT very challenging since you 
> basically have to handle bouncing any possible buffer.  Since the L1 
> table occurs immediately after the header, there's really no room to 
> play any kind of tricks to add features.

That's a good point actually. I didn't remember that.

Kevin