Re: block snapshot issue with RBD

[Fiona Ebner]

Hi,

Am 28.05.24 um 20:19 schrieb Jin Cao:
> Hi Ilya
> 
> On 5/28/24 11:13 AM, Ilya Dryomov wrote:
>> On Mon, May 27, 2024 at 9:06 PM Jin Cao <jojing64@gmail.com> wrote:
>>>
>>> Supplementary info: VM is paused after "migrate" command. After being
>>> resumed with "cont", snapshot_delete_blkdev_internal works again, which
>>> is confusing, as disk snapshot generally recommend I/O is paused, and a
>>> frozen VM satisfy this requirement.
>>
>> Hi Jin,
>>
>> This doesn't seem to be related to RBD.  Given that the same error is
>> observed when using the RBD driver with the raw format, I would dig in
>> the direction of migration somehow "installing" the raw format (which
>> is on-disk compatible with the rbd format).
>>
> 
> Thanks for the hint.
> 
>> Also, did you mean to say "snapshot_blkdev_internal" instead of
>> "snapshot_delete_blkdev_internal" in both instances?
> 
> Sorry for my copy-and-paste mistake. Yes, it's snapshot_blkdev_internal.
> 
> -- 
> Sincerely,
> Jin Cao
> 
>>
>> Thanks,
>>
>>                  Ilya
>>
>>>
>>> -- 
>>> Sincerely
>>> Jin Cao
>>>
>>> On 5/27/24 10:56 AM, Jin Cao wrote:
>>>> CC block and migration related address.
>>>>
>>>> On 5/27/24 12:03 AM, Jin Cao wrote:
>>>>> Hi,
>>>>>
>>>>> I encountered RBD block snapshot issue after doing migration.
>>>>>
>>>>> Steps
>>>>> -----
>>>>>
>>>>> 1. Start QEMU with:
>>>>> ./qemu-system-x86_64 -name VM -machine q35 -accel kvm -cpu
>>>>> host,migratable=on -m 2G -boot menu=on,strict=on
>>>>> rbd:image/ubuntu-22.04-server-cloudimg-amd64.raw -net nic -net user
>>>>> -cdrom /home/my/path/of/cloud-init.iso -monitor stdio
>>>>>
>>>>> 2. Do block snapshot in monitor cmd: snapshot_delete_blkdev_internal.
>>>>> It works as expected: the snapshot is visable with command`rbd snap ls
>>>>> pool_name/image_name`.
>>>>>
>>>>> 3. Do pseudo migration with monitor cmd: migrate -d
>>>>> exec:cat>/tmp/vm.out
>>>>>
>>>>> 4. Do block snapshot again with snapshot_delete_blkdev_internal, then
>>>>> I get:
>>>>>      Error: Block format 'raw' used by device 'ide0-hd0' does not
>>>>> support internal snapshots
>>>>>
>>>>> I was hoping to do the second block snapshot successfully, and it
>>>>> feels abnormal the RBD block snapshot function is disrupted after
>>>>> migration.
>>>>>
>>>>> BTW, I get the same block snapshot error when I start QEMU with:
>>>>>       "-drive format=raw,file=rbd:pool_name/image_name"
>>>>>
>>>>> My questions is: how could I proceed with RBD block snapshot after the
>>>>> pseudo migration?
> 
> 

I bisected this issue to d3007d348a ("block: Fix crash when loading
snapshot on inactive node").

> diff --git a/block/snapshot.c b/block/snapshot.c
> index ec8cf4810b..c4d40e80dd 100644
> --- a/block/snapshot.c
> +++ b/block/snapshot.c
> @@ -196,8 +196,10 @@ bdrv_snapshot_fallback(BlockDriverState *bs)
>  int bdrv_can_snapshot(BlockDriverState *bs)
>  {
>      BlockDriver *drv = bs->drv;
> +
>      GLOBAL_STATE_CODE();
> -    if (!drv || !bdrv_is_inserted(bs) || bdrv_is_read_only(bs)) {
> +
> +    if (!drv || !bdrv_is_inserted(bs) || !bdrv_is_writable(bs)) {
>          return 0;
>      }
>  

So I guess the issue is that the blockdev is not writable when
"postmigrate" state?

Best Regards,
Fiona