Re: [Qemu-devel] [RFC PATCH 0/4] savevm: save vmsate with fixed size

[Kevin Wolf]

Am 01.03.2013 um 03:35 hat Wenchao Xia geschrieben:
> 于 2013-2-28 18:50, Kevin Wolf 写道:
> >Am 28.02.2013 um 09:09 hat Wenchao Xia geschrieben:
> >>This version have following limitation:
> >>   1 in patch 3 only dirty page got written, clean page is not touched, so
> >>it will have trouble when savevm to an old internal snapshot, which
> >>will be fixed later if this approach seems OK.
> >
> >Basically you need a bdrv_zero_vmstate(), right? I think this would
>   Yes, an API to initialize the data at the beginning, or just write 4K
> zero in the progress....
> 
> >actually be a bug fix, because snapshots might today get references to
> >unused VM state clusters that are just leftovers from the last snapshot.
> >
>   In a qcow2 file that have snapA, if user type "savevm snapA", then
> qemu will delete old snapA and then create new snapA.
>   Do you mean that new snapA and old snapA may use the same cluster
> that is not cleaned up as zeros? I guess this brings no trouble to old
> stream savevm, but will brings trouble to plane savevm in this patch.
> If so, I think yes this bug fix can solve the problem.

The scenario I'm thinking of is something like:

1. (qemu) savevm A
2. (qemu) quit
3. qemu-img snapshot -c B test.qcow2
4. qemu-img snapshot -d A test.qcow2

Step 1 creates a snapshot from a running VM, so it writes a lot of VM
state data to the image. Step 3 creates another snapshot, however
outside of a running qemu, so without VM state. It wrongly gets a
reference to all VM state clusters of A, which haven't been overwritten
or discarded since snapshot A was taken. When deleting A in step 4, the
clusters cannot be freed because they are still referenced by B (which
doesn't need them at all)

Kevin