Re: [Qemu-devel] [PATCH 0/3]: BLOCK_WATERMARK QMP event

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/09/2010 05:22 PM, Luiz Capitulino wrote:
> On Tue, 09 Mar 2010 17:08:33 -0600
> Anthony Liguori<anthony@codemonkey.ws>  wrote:
>
>    
>> On 03/09/2010 04:53 PM, Luiz Capitulino wrote:
>>      
>>>    Hi,
>>>
>>>    This series is based on a previous series submitted by Uri Lublin:
>>>
>>> http://lists.gnu.org/archive/html/qemu-devel/2009-03/msg00864.html
>>>
>>>    Details on the patches, except for this question: does it make sense to have
>>> a 'low' watermark for block devices?
>>>
>>>    I think it doesn't, then the event (and the monitor accompanying command)
>>> should be called BLOCK_HIGH_WATERMARK. But this makes the event very
>>> unflexible, so I have called it BLOCK_WATERMARK and added parameters for the
>>> high/low watermark type.
>>>
>>>        
>> The alternative way to implement this is for a management tool to just
>> poll the allocated disk size periodically.
>>
>> It's no more/less safe than generating an event on a "watermark" because
>> the event is still racy with respect to a guest that's writing very
>> quickly to the disk.
>>      
>   The argument against polling is not only about possible races, but
> it's something very inefficient to do as the condition you're polling
> for may never happen but you're wasting cpu resources for it.
>    

No, you just move the polling into qemu in the block I/O path.  It's now 
"polling" for whether a watermark has been hit.

The problem with this functionality is that it's extremely tied to a 
very specific management policy.  It's not at all generic.

Polling is an entirely legitimate thing for a management tool to do and 
we shouldn't introduce events that don't make sense just to avoid polling.

Regards,

Anthony Liguori