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Re: [Qemu-devel] [PATCH v13 07/14] block/mirror: Improve progress report

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v13 07/14] block/mirror: Improve progress report
Date: Thu, 23 Oct 2014 07:10:42 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

On 10/23/2014 04:52 AM, Kevin Wolf wrote:
> Am 22.10.2014 um 14:51 hat Max Reitz geschrieben:
>> Instead of taking the total length of the block device as the block
>> job's length, use the number of dirty sectors. The progress is now the
>> number of sectors mirrored to the target block device. Note that this
>> may result in the job's length increasing during operation, which is
>> however in fact desirable.
> More importantly, because it might surprise management tools, is that
> the progress (as in offset/len) can actually decrease now.

Decreased progress is a GOOD thing - it tells management that the guest
is causing I/O faster than the mirroring can keep up with, and that it
might be time to turn on some throttling if the mirror is going to have
a chance at converging.  The old code didn't have any correlation to
progress complete vs work remaining, making it harder to accurately
estimate how long a job still has to go.

> I can't say whether that creates any problem, I'll rely on Eric's
> Reviewed-by for that.

Libvirt has already documented that for all jobs (including block jobs),
progress is estimated by offset/total, but that total need not be
constant between two consecutive calls, precisely for the behavior that
this patch moves toward.

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

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