Re: [Qemu-devel] savevm/loadvm

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 11/02/2013 01:16 AM, Max Reitz wrote:
> 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?


Yes, this works now, thanks!


> 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.
>>
>>
>>
> 


-- 
Alexey