Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change

Dor Laor
2009-02-14T22:28:36Z

Marc Bevand wrote:
> On Fri, Feb 13, 2009 at 8:23 AM, Jamie Lokier <jamie@shareable.org> wrote:
>
>> Marc.. this is quite a serious bug you've reported. Is there a
>> reason you didn't report it earlier?
>>
>
> Because I only started hitting that bug a couple weeks ago after
> having upgraded to a buggy kvm version.
>
>
>> Is there a way to restructure the code and/or how it works so it's
>> more clearly correct?
>>
>
> I am seriously concerned about the general design of qcow2. The code
> base is more complex than it needs to be, the format itself is
> susceptible to race conditions causing cluster leaks when updating
> some internal datastructures, it gets easily fragmented, etc.
>
> I am considering implementing a new disk image format that supports
> base images, snapshots (of the guest state), clones (of the disk
> content); that has a radically simpler design & code base; that is
> always consistent "on disk"; that is friendly to delta diffing (ie.
> space-efficient when used with ZFS snapshots or rsync); and that makes
> use of checksumming & replication to detect & fix corruption of
> critical data structures (ideally this should be implemented by the
> filesystem, unfortunately ZFS is not available everywhere :D).
>
> I believe the key to achieve these (seemingly utopian) goals is to
> represent a disk "image" as a set of sparse files, 1 per
> snapshot/clone.
>
Both qcow2 and vmdk have the ability to keep 'external' snapshots.
In addition to what you wrote, qcow2 is missing journal for its meta
data and
also performs poorly because of complex meta data and sync calls.

We might use vmdk format or VHD as a base for the future high
performing, safe
image format for qemu
-dor
> -marc
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