Re: [Qemu-block] [Qemu-devel] RFC: use case for adding QMP, block jobs & multiple exports to qemu-nbd ?

[Stefan Hajnoczi]

On Fri, Nov 03, 2017 at 02:00:46PM +0800, Fam Zheng wrote:
> On Thu, 11/02 12:02, Daniel P. Berrange wrote:
> > One alternative approach to doing this would be to suggest that we should
> > instead just spawn qemu-system-x86_64 with '--machine none' and use that
> > as a replacement for qemu-nbd, since it already has a built-in NBD server
> > which can do many exports at once and arbitrary block jobs.
> 
> Here is a crazy idea from KVM Forum discussions that may relate, so mention it
> here: we could move the QEMU block layer to a separate program and guest can 
> use
> vhost-user-{blk,scsi} for I/O. It is something like this:
> 
> 
>    master-disk1.qcow2  (qemu-nbd)
>           ^
>           |  backing
>           |
>    cache-disk1.qcow2   (qemu-vhost)     <-------------.
>           ^                                           |
>           |  backing                                  |  backing
>           |                                           |
>           +-  vm-a-disk1.qcow2   (qemu-vhost)         +-  vm-a-disk2.qcow2   
> (qemu-vhost)
>                     ^                                             ^
>                     |  vhost-user-blk                             |  
> vhost-user-blk
>                     |                                             |
>                     +- VM-1 (qemu-system-XXX)                     +- VM-2 
> (qemu-system-XXX)
> 
> 
> So on the compute node, there will be N qemu-system-XXX processes (where N is
> the number of VMs) and 1 qemu-vhost process.
> 
> The hypothetical qemu-vhost program needs to support QMP as well and it runs 
> the
> COR/mirroring jobs from master disk to cache disk, just like what you propose 
> to
> do with the extended qemu-nbd. The only difference is replacing the local NBD
> with vhost-user, which is more efficient.

The vhost-user part can be added later.  I think the first step is
whether to add a QMP interface to qemu-nbd or use -machine none.

Stefan

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