Re: [Qemu-devel] [PATCH v9 06/14] block/mirror: Improve progress report

[Eric Blake]

On 07/09/2014 03:24 PM, Paolo Bonzini wrote:
> Il 07/07/2014 21:13, Eric Blake ha scritto:
>> On 07/05/2014 11:47 AM, Max Reitz wrote:
>>> 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.
>>>
>>> Signed-off-by: Max Reitz <address@hidden>
>>> ---
>>>  block/mirror.c | 34 ++++++++++++++++++++++------------
>>>  1 file changed, 22 insertions(+), 12 deletions(-)
>>
>> Reviewed-by: Eric Blake <address@hidden>
> 
> This is an API change... IIUC the length can become bigger than the
> underlying device's size.  Eric, how would libvirt expose this to
> clients and what are the chances that they get confused?

Libvirt has already documented that a job size is unrelated to the block
device size, that it is only an approximation to completion, and that
the completion number may change during operation.  The only hard and
fast rule is that the job is finished when the two counters are equal.
I recommended this change precisely because the new semantics are better
than the old, including how libvirt exposes the numbers to the end user
- if the end parameter continues to grow more than the current
parameter, it is DESIRABLE to expose that as sign that the guest is
dirtying pages fast enough to cause problems in converging the block job.

http://libvirt.org/html/libvirt-libvirt.html#virDomainBlockJobInfo
"The following fields provide an indication of block job progress. @cur
indicates the current position and will be between 0 and @end. @end is
the final cursor position for this operation and represents completion.
To approximate progress, divide @cur by @end."

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

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