Re: [Qemu-devel] savevm/loadvm

[Max Reitz]

Hi,

Sorry I'm just now replying to this. I ran into the same issue (and
another one) and it should be fixed by the upstream commits
eedff66f21e542650d895801549ce05ac108278b and
6e13610aa454beba52944e8df6d93158d68ab911. Those have been merged to
master yesterday, so could you re-build qemu from master and try again?


Kind regards,

Max


On 08.10.2013 10:40, Alexey Kardashevskiy wrote:
> Hi!
>
> I need the community help with savevm/loadvm.
>
> I run QEMU like this:
>
> ./qemu-system-ppc64 \
>  -drive file=virtimg/fc19_16GB.qcow2 \
>  -nodefaults \
>  -m "2048" \
>  -machine "pseries" \
>  -nographic \
>  -vga "none" \
>  -enable-kvm
>
>
> The disk image is an 16GB qcow2 image.
>
> Now I start the guest and do "savevm 1" and "loadvm 1" from the qemu
> console. Everything works. Then I exit qemu, make sure that the snapshot is
> there and run QEMU as above plus "-loadvm 1". It fails with:
>
> qemu-system-ppc64: qcow2: Loading snapshots with different disk size is not
> implemented
> qemu-system-ppc64: Error -95 while activating snapshot '2' on 'scsi0-hd0'
>
> The check is added by commit 90b277593df873d3a2480f002e2eb5fe1f8e5277
> "qcow2: Save disk size in snapshot header".
>
> As I cannot realize the whole idea of the patch, I looked a bit deeper.
> This is the check:
>
> int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
> {
> [...]
>     if (sn->disk_size != bs->total_sectors * BDRV_SECTOR_SIZE) {
>         error_report("qcow2: Loading snapshots with different disk "
>             "size is not implemented");
>         ret = -ENOTSUP;
>         goto fail;
>     }
>
>
> My understanding of the patch was that the disk_size should remain 16GB
> (0x4.0000.0000) as it uses bs->total_sectors and never changes it. And
> bs->growable is 0 for qcow2 image because it is not really growable. At
> least the total_sectors value from the qcow2 file header does not change
> between QEMU starts.
>
> However qcow2_save_vmstate() sets bs->growable to 1 for a short time
> (commit 178e08a58f40dd5aef2ce774fe0850f5d0e56918 from 2009) and this
> triggers a branch in bdrv_co_do_writev() which changes bs->total_sectors.
> So when QEMU writes snapshots to the file, the disk_size field of a
> snapshot has bigger value (for example 0x4.007b.8180).
>
> And the check above fails. It does not fail if to do "loadvm"
> _in_the_same_run_ after "savevm" because QEMU operates with the updated
> bs->total_sectors.
>
> What the proper fix would be? Or it is not a bug at all and I should be
> using something else for "-loadvm"? Thanks.
>
>
>