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

[Eric Blake]

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.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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