[Qemu-devel] Re: [PATCH 1/2] block: Add wr_highest_sector blockstat

[Anthony Liguori <anthony@codemonkey.ws>]

On 04/28/2010 12:04 PM, Luiz Capitulino wrote:
> On Wed, 28 Apr 2010 11:01:12 -0500
> Anthony Liguori<anthony@codemonkey.ws>  wrote:
>
>    
>> On 04/28/2010 10:56 AM, Kevin Wolf wrote:
>>      
>>> This adds the wr_highest_sector blockstat which implements what is generally
>>> known as the high watermark. It is the highest offset of a sector written to
>>> the respective BlockDriverState since it has been opened.
>>>
>>> The query-blockstat QMP command is extended to add this value to the result,
>>> and also to add the statistics of the underlying protocol in a new "parent"
>>> field. Note that to get the "high watermark" of a qcow2 image, you need to look
>>> into the wr_highest_sector field of the parent (which can be a file, a
>>> host_device, ...). The wr_highest_sector of the qcow2 BlockDriverState itself
>>> is the highest offset on the _virtual_ disk that the guest has written to.
>>>
>>> Signed-off-by: Kevin Wolf<kwolf@redhat.com>
>>>
>>>        
>> I see, you did print out stats for each layer.
>>
>> I don't think we should take 2/2.  I don't mind QMP having more features
>> than the user monitor.
>>      
>   I don't either, but Kevin has said to me that this information is also good
> for the user Monitor.
>
>   The real question here is whether or not we're going to stop supporting
> stability for the user Monitor and if so, when we'll break it.
>
>   An arguable reasonable policy would be to try to maintain stability for
> existing commands. In this specific case, 'info blockstats' is used by
> libvirt afaik. So breaking it would mean that older libvirt versions won't
> be able to talk to newer qemu (taking libvirt just as real known example).
>    

I think we should try our best to maintain compatibility.  In this case, 
this change would break any non-QMP version of libvirt so it would be 
pretty painful for users.  That's why I'm inclined to not take.

It would be reasonable to add a new info command for the user monitor if 
the functionality is desirable.

Regards,

Anthony Liguori