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

[Avi Kivity]

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:

guest: write(sector=0, count=1) (A)
guest: write(sector=1, count=1) (B)
qemu: read(sector=0, count=16) (A)
qemu: submit aio write (sector=0, count=16) (A)
qemu: read(sector=0, count=16) (B)
qemu: submit aio write (sector=0, count=16) (B) - contains data from 
before A's write

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.

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