Re: [PATCH v2 11/20] qapi: backup: add x-max-chunk and x-max-workers parameters

[Max Reitz <mreitz@redhat.com>]

On 22.10.20 22:35, Vladimir Sementsov-Ogievskiy wrote:
> 22.07.2020 15:22, Max Reitz wrote:
>> On 01.06.20 20:11, Vladimir Sementsov-Ogievskiy wrote:
>>> Add new parameters to configure future backup features. The patch
>>> doesn't introduce aio backup requests (so we actually have only one
>>> worker) neither requests larger than one cluster. Still, formally we
>>> satisfy these maximums anyway, so add the parameters now, to facilitate
>>> further patch which will really change backup job behavior.
>>>
>>> Options are added with x- prefixes, as the only use for them are some
>>> very conservative iotests which will be updated soon.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>   qapi/block-core.json      |  9 ++++++++-
>>>   include/block/block_int.h |  7 +++++++
>>>   block/backup.c            | 21 +++++++++++++++++++++
>>>   block/replication.c       |  2 +-
>>>   blockdev.c                |  5 +++++
>>>   5 files changed, 42 insertions(+), 2 deletions(-)
>>>
> 
> [..]
> 
>>> @@ -422,6 +436,11 @@ BlockJob *backup_job_create(const char *job_id,
>>> BlockDriverState *bs,
>>>       if (cluster_size < 0) {
>>>           goto error;
>>>       }
>>> +    if (max_chunk && max_chunk < cluster_size) {
>>> +        error_setg(errp, "Required max-chunk (%" PRIi64") is less
>>> than backup "
>>
>> (missing a space after PRIi64)
>>
>>> +                   "cluster size (%" PRIi64 ")", max_chunk,
>>> cluster_size);
>>
>> Should this be noted in the QAPI documentation?
> 
> Hmm.. It makes sense, but I don't know what to write: should be >= job
> cluster_size? But then I'll have to describe the logic of
> backup_calculate_cluster_size()...

Isn’t the logic basically just “cluster size of the target image file
(min 64k)”?  The other cases just cover error cases, and they always
return 64k, which would effectively be documented by stating that 64k is
the minimum.

But in the common cases (qcow2 or raw), we’ll never get an error anyway.

I think it’d be good to describe the cluster size somewhere, because
otherwise I find it a bit malicious to throw an error at the user that
they could not have anticipated because they have no idea what valid
values for the parameter are (until they try).

>>  (And perhaps the fact
>> that without copy offloading, we’ll never copy anything bigger than one
>> cluster at a time anyway?)
> 
> This is a parameter for background copying. Look at
> block_copy_task_create(), if call_state has own max_chunk, it's used
> instead of common copy_size (derived from cluster_size). But at a moment
> of this patch background process through async block-copy is not  yet
> implemented, and the parameter doesn't work, which is described in
> commit message.

Ah, OK.  Right.

Max