[Qemu-devel] Re: KVM call agenda for Apr 27

[Anthony Liguori]

On 04/27/2010 03:14 AM, Avi Kivity wrote:
> On 04/27/2010 01:36 AM, Anthony Liguori wrote:
> > A few comments:
> >
> > 1) The problem was not block watermark itself but generating a 
> > notification on the watermark threshold.  It's a heuristic and should 
> > be implemented based on polling block stats. 
>
> Polling for an event that never happens is bad engineering.  What 
> frequency do you poll?  you're forcing the user to make a lose-lose 
> tradeoff.
>
> > Otherwise, we'll be adding tons of events to qemu that we'll struggle 
> > to maintain.
>
> That's not a valid reason to reject a user requirement.  We may argue 
> the requirement is bogus, or that the suggested implementation is 
> wrong and point in a different direction, but saying that we may have 
> to add more code in the future due to other requirements is ... well I 
> can't find a word for it.

Polling is the best solution because it offers the most flexibility.  
Baking the heuristic into qemu just removes flexibility for all consumers.

> > 2) A block plugin doesn't solve the problem if it's just at the 
> > BlockDriverState level because it can't interact with qcow2.
>
> Why not?  We have a layered model.  guest -> qcow2 -> plugin (sends 
> event) -> raw-posix.  Just need to insert the plugin at the 
> appropriate layer.

All of the qcow2 information is static to the qcow2 driver and I don't 
think changing that for plugins is a good idea.

> > 3) For general block plugins, it's probably better to tackle 
> > userspace block devices.  We have CUSE and FUSE already, a BUSE is a 
> > logical conclusion.
>
> We also have an nbd client.
>
> Here's another option:  an nbd-like protocol that remotes all 
> BlockDriver operations except read and write over a unix domain 
> socket.  The open operation returns an fd (SCM_RIGHTS strikes again) 
> that is used for read and write.  This can be used to implement 
> snapshots over LVM, for example.

How does it address the watermark problem?

Regards,

Anthony Liguori