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

[Anthony Liguori]

On 03/11/2010 08:58 AM, Avi Kivity wrote:
> On 03/11/2010 04:19 PM, Anthony Liguori wrote:
> > > But honestly, while I do understand your point, this feels like a hack
> > > to work around shortcomings of an interface. So what we need to decide
> > > is which criterion outweighs the other in practice.
> >
> > If I understand the use case correctly, what this really boils down 
> > to is that you want to create a growable image on top of a 
> > non-growable device.  The management tool then deals with growth 
> > using this interface.
> >
> > I'm somewhat inclined to suggest that the proper way to support this 
> > is to teach qemu how to grow the LVM volume like it would grow any 
> > normal file.
>
> Interesting.  Could use a helper or helper.so, or a new block format 
> driver (block/lvm.c).

I've suggested plugins for the block drivers in the past.  I don't think 
it was well received.

>   Still, if management is actually interested in the change, not just 
> to grow the volume (perhaps to handle out-of-space, or to check 
> against a global disk low condition), the helper/driver then needs to 
> communicate with the management system, which is awkward.

The management system can poll information from the block driver.  
Polling is not always evil :-)

Regards,

Anthony Liguori

> I guess it's workable though.