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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Tue, 9 Mar 2010 20:22:03 -0300
Luiz Capitulino <lcapitulino@redhat.com> 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.

 Also note that this applies for other events and if this becomes the
standard mode of operation, the end result is that we're delegating
event generation for the management tools.