Re: [Qemu-block] [PATCH] backup: allow target without .bdrv_get_info

[Denis V. Lunev]

On 02/14/2017 06:05 PM, Fam Zheng wrote:
> On Tue, 02/14 17:48, Vladimir Sementsov-Ogievskiy wrote:
>> Currently backup to nbd target is broken, as nbd doesn't have
>> .bdrv_get_info realization.
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>> ---
>>
>> Hi all!
>>
>> Since commit
>>
>> commit 4c9bca7e39a6e07ad02c1dcde3478363344ec60b
>> Author: John Snow <address@hidden>
>> Date:   Thu Feb 25 15:58:30 2016 -0500
>>
>>     block/backup: avoid copying less than full target clusters
>>
>> backup to nbd target is broken, we have "Couldn't determine the cluster size 
>> of
>> the target image".
>>
>> Proposed NBD protocol extension - NBD_OPT_INFO should finally solve this 
>> problem.
>> But until it is not realized, we need allow backup to nbd target due to 
>> backward
>> compatibility.
>>
>> Furthermore, is it entirely ok to disallow backup if bds lacks 
>> .bdrv_get_info?
>> Which behavior should be default: to fail backup or to use default cluster 
>> size?
>>
>>  block/backup.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/backup.c b/block/backup.c
>> index ea38733..095a390 100644
>> --- a/block/backup.c
>> +++ b/block/backup.c
>> @@ -638,7 +638,10 @@ BlockJob *backup_job_create(const char *job_id, 
>> BlockDriverState *bs,
>>       * backup cluster size is smaller than the target cluster size. Even for
>>       * targets with a backing file, try to avoid COW if possible. */
>>      ret = bdrv_get_info(target, &bdi);
>> -    if (ret < 0 && !target->backing) {
>> +    if (ret == -ENOTSUP) {
>> +        /* Cluster size is not defined */
>> +        job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
>> +    } else if (ret < 0 && !target->backing) {
>>          error_setg_errno(errp, -ret,
>>              "Couldn't determine the cluster size of the target image, "
>>              "which has no backing file");
>> -- 
>> 1.8.3.1
>>
> Target not having .bdrv_get_info means 'some sane assumptions are okay' to 
> me, so
>
> Reviewed-by: Fam Zheng <address@hidden>
>
> (But we are going to have a .bdrv_get_info after NBD_OPT_INFO, right?)
>
yes, sooner or later :)