[libvirt] [PATCH for 5.3.0] storage_backend_rbd: Don't fail iterating over snapshots

Michal Privoznik mprivozn at redhat.com
Mon Apr 29 14:09:39 UTC 2019


On 4/29/19 3:13 PM, Jason Dillaman wrote:
> On Mon, Apr 29, 2019 at 8:57 AM Michal Privoznik <mprivozn at redhat.com> wrote:
>>
>> When cloning an RBD volume we try to find a snapshot which is
>> not different to the image we're trying to clone. This boils down
>> to calling rbd_diff_iterate() or rbd_diff_iterate2() on systems
>> with newer ceph. These two are passed a callback -
>> virStorageBackendRBDIterateCb() which mistakenly states that if
>> a negative value is returned from the callback, the iteration is
>> stopped but no error is reported. Well, that is not the case.
>> I've looked into librbd sources and found that a negative value
>> does mean stop of iteration but it also means an error. That's
>> why any attempt to clone an RBD image results in the following
>> error:
>>
>>    virsh # vol-clone --pool rbd_image_root coreos_2023 coreos00.disk
>>    error: Failed to clone vol from coreos_2023
>>    error: failed to iterate RBD snapshot coreos_2023 at base: Operation not
>>    permitted
>>
>> Reported on libvirt-users list:
>>
>> https://www.redhat.com/archives/libvirt-users/2019-April/msg00060.html
>>
>> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>> ---
>>   src/storage/storage_backend_rbd.c | 7 +------
>>   1 file changed, 1 insertion(+), 6 deletions(-)
>>
>> diff --git a/src/storage/storage_backend_rbd.c b/src/storage/storage_backend_rbd.c
>> index f8c968e682..3fe188551c 100644
>> --- a/src/storage/storage_backend_rbd.c
>> +++ b/src/storage/storage_backend_rbd.c
>> @@ -1031,14 +1031,9 @@ virStorageBackendRBDIterateCb(uint64_t offset ATTRIBUTE_UNUSED,
>>   {
>>       /*
>>        * Just set that there is a diff for this snapshot, we do not care where
>> -     *
>> -     * When it returns a negative number the rbd_diff_iterate() function will stop
>> -     *
>> -     * That's why we return -1, meaning that there is a difference and we can stop
>> -     * searching any further.
>>        */
>>       *(int*) arg = 1;
>> -    return -1;
>> +    return 0;
>>   }
> 
> You could also tweak "virStorageBackendRBDSnapshotFindNoDiff" to
> ignore the error if "diff > 0". That would still allow it to fail-fast
> once a delta is found.

Good catch. Let me post a v2.

Michal




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