Re: [Qemu-devel] Strange virtio regression on mainline and stable-0.10

[Kevin Wolf]

Avi Kivity schrieb:
> Avi Kivity wrote:
>> Running the Fedora 10 installer on a virtio disk on current master and 
>> on v0.10.3 will cause the installer to complain when mounting the 
>> freshly formatted filesystems.
> 
> The problem is that qcow2 does a read-modify-write on 
> non-cluster-aligned writes.  So the following sequence triggers the bug:
> [...]
> 
> This could be solved by maintaining a hash table of refcounted RMW 
> copies for the disk.  When reading for a RMW, look up the hash table, if 
> there's a copy there, use it instead of reading it yourself.
> 
> We should also avoid the RMW for non-compressed, non-encrypted clusters, 
> as virtually ALL writes will be misaligned.

I don't think there is a RMW except for the COW case which is
unavoidable and obviously happens only once for each cluster. Do you see
any other places where this happens?

Kevin