Re: [Qemu-devel] [PATCH 1/1] mirror: restore offset after zeroing out the image

["Denis V. Lunev" <den@openvz.org>]

On 01/30/2017 08:16 PM, Eric Blake wrote:
> On 01/30/2017 04:22 AM, Denis V. Lunev wrote:
>> If explicit zeroing out before mirroring is required for the target image,
>> it moves the block job offset counter to EOF, then offset and len counters
>> count the image size twice.
>>
>> There is no harm but confusing stats (e.g. for 1G image the completion
>> counter starts from 1G and increases to 2G)
>>
>> The patch fixed that problem by resetting the offset counter.
> Counters are explicitly documented NOT tied to disk length; they are
> merely estimates of proportional completion.  I'm not sure if this makes
> the numbers jump backwards from the observer's viewpoint, but if you can
> ever spot 1g/1g right before rewinding to 0g/1g (where pre-patch could
> see 1g/2g), then that is a reason to not take this patch.  On the other
> hand, your argument that the pre-patch behavior progressing towards 2g
> has a very fast progression from 0-1g/2g, and then a much slower
> 1g-2g/2g, which makes the estimate of percent completion skewed, while a
> newer progression of 0-1g/1g is more realistic, may have some merit.
>
> I'm not sold on this patch yet, but stronger arguments in the commit
> message may sway me.
>
>> +++ b/tests/qemu-iotests/094.out
>> @@ -3,9 +3,9 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
>>  Formatting 'TEST_DIR/source.IMGFMT', fmt=IMGFMT size=67108864
>>  {"return": {}}
>>  {"return": {}}
>> -{"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "BLOCK_JOB_READY", "data": {"device": "src", "len": 67108864, "offset": 67108864, "speed": 0, "type": "mirror"}}
>> +{"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "BLOCK_JOB_READY", "data": {"device": "src", "len": 0, "offset": 0, "speed": 0, "type": "mirror"}}
>>  {"return": {}}
> This part of the change is scary - a ready event showing 0/0 HAS been
> known to confuse libvirt in the past.  Qemu should NEVER advertise a
> ready event with 0/0, it should at least be 1/1 (because of the number
> of clients that have workarounds to deal with older qemu behavior on 0/0
> and which might misbehave if we ever issue that again).
>
>> -{"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "src", "len": 67108864, "offset": 67108864, "speed": 0, "type": "mirror"}}
>> +{"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "src", "len": 0, "offset": 0, "speed": 0, "type": "mirror"}}
> So NACK to the patch as currently written, but not necessarily to the
> idea if you can give better progress numbers and never reach the state
> of BLOCK_JOB_READY and BLOCK_JOB_COMPLETED with a 0/0 length/offset.
>
ok. fair enough. Thank you for the review.

Will it be better to (somehow) skip progressing below using
some condition during mirror_dirty_init() stage?

static void mirror_iteration_done(MirrorOp *op, int ret)
{
    .....

    if (ret >= 0) {
        if (s->cow_bitmap) {
            bitmap_set(s->cow_bitmap, chunk_num, nb_chunks);
        }
        s->common.offset += (uint64_t)op->nb_sectors * BDRV_SECTOR_SIZE;
<---- specifically this progressing
    }

Den