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

[Luiz Capitulino]

On Tue, 09 Mar 2010 17:08:33 -0600
Anthony Liguori <address@hidden> 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.