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

[Anthony Liguori]

On 09/10/2010 07:06 AM, Avi Kivity wrote:
>  On 09/10/2010 02:43 PM, Stefan Hajnoczi wrote:
> > > > and/or enterprise storage.
> > > That doesn't eliminate undiscovered errors (they can still come from 
> > > the
> > > transport).
> > Eliminating silent data corruption is currently not a goal for any
> > disk image format I know of.  For filesystems, I know that ZFS and
> > btrfs will try to detect corruption using data checksumming.
> >
> > The guest filesystem, the disk image format, or the host filesystem
> > could do checksumming.  The hypervisor should keep out of the way in
> > the interest of performance and emulation fidelity.  Why does
> > checksumming need to be done in the image format?  Isn't the choice
> > between host and guest filesystem checksumming already enough?
>
> You're correct about the data.  It's better to do it at the end-point 
> in any case.
>
> The metadata is something else - an error in a cluster table is 
> magnified so it is likely to cause the loss of an entire image, and 
> there's nothing the guest can do about it.  btrfs duplicates metadata 
> to avoid this (but if we have btrfs underneath, we can just use raw).

What it really comes down to is that checksumming is a filesystem 
feature that requires a sophisticated way of handling metadata which 
puts it beyond the scope of what an image format should be.

The point of an image format is to make it a filesystem from 10 years 
ago in terms of sophistication and leave the cutting edge file system 
research to file system developers.

Regards,

Anthony Liguori