Re: [Qemu-devel] [RFC V4 00/30] QCOW2 deduplication

[Benoît Canet]

I think I can easily add a "verify" option at image creation.
This way the code would read the cluster already on disk and compare it with
the cluster to write.
If there are different it would print some debug message and return -EIO to the
upper layers.

> Le Wednesday 02 Jan 2013 à 11:01:04 (-0700), Eric Blake a écrit :
> On 01/02/2013 10:33 AM, Benoît Canet wrote:
> >> How does this code handle hash collisions, and do you have some regression
> >> tests that purposefully create a dedup hash collision, and verify that the
> >> 'right thing' happens?
> > 
> > The two hash function that can be used are cryptographics and not broken 
> > yet.
> > So nobody knows how to generate a collision.
> 
> I can understand that it is hard to write a test for two distinct data
> sectors hashing to the same value, but perhaps it's worth including a
> debug-only hash algorithm that intentionally generates collisions, just
> to prove that you handle them correctly.  De-duplicating collided data,
> while unlikely, is still a case of data loss that not everyone is happy
> to risk.
> 
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>