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

[Avi Kivity]

On 04/27/2010 11:48 AM, Dor Laor wrote:
> > 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.
>
>
> Why w/o read/writes? 

To avoid the copying.

> the watermark code needs them too (as info, not the actual buffer).

Yeah.  It works for lvm snapshots, not for watermarks.

> IMHO the whole thing is way over engineered:
>  a) Having another channel into qemu is complicating management
>     software. Isn't the monitor should be the channel? Otherwise we'll
>     need to create another QMP (or nbd like Avi suggest) for these
>     actions. It's extra work for mgmt and they will have hard time to
>     understand events interleaving of the various channels

block layer plugins allow intercepting all interesting block layer 
events, not just write-past-a-watermark, and allow actions based on 
those events.  It's a more general solution.

>  b) How the plugins are defined? Is it scripts? Binaries? Do they open
>     their own sockets?

Shared objects.


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