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
