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

[Avi Kivity <avi@redhat.com>]

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.

>
> 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.

>
> 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.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.