[Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support

[Kevin Wolf]

Am 12.10.2010 17:22, schrieb Anthony Liguori:
> On 10/12/2010 10:08 AM, Kevin Wolf wrote:
>>   Otherwise we might destroy data that isn't
>> even touched by the guest request in case of a crash.
>>    
> 
> The failure scenarios are either that the cluster is leaked in which 
> case, the old version of the data is still present or the cluster is 
> orphaned because the L2 entry is written, in which case the old version 
> of the data is present.

Hm, how does the latter case work? Or rather, what do mean by "orphaned"?

> Are you referring to a scenario where the cluster is partially written 
> because the data is present in the write cache and the write cache isn't 
> flushed on power failure?

The case I'm referring to is a COW. So let's assume a partial write to
an unallocated cluster, we then need to do a COW in pre/postfill. Then
we do a normal write and link the new cluster in the L2 table.

Assume that the write to the L2 table is already on the disk, but the
pre/postfill data isn't yet. At this point we have a bad state because
if we crash now we have lost the data that should have been copied from
the backing file.

If we can't guarantee that a new cluster is all zeros, the same happens
without a backing file. So as soon as we start reusing freed clusters,
we get this case for all QED images.

Kevin