Re: [Qemu-devel] [RFC PATCH 1/1] ceph/rbd block driver for qemu-kvm

[Anthony Liguori]

On 05/25/2010 08:57 AM, Avi Kivity wrote:
> On 05/25/2010 04:54 PM, Anthony Liguori wrote:
> > On 05/25/2010 08:36 AM, Avi Kivity wrote:
> > > We'd need a kernel-level generic snapshot API for this eventually.
> > >
> > > > or (2) implement BUSE to complement FUSE and CUSE to enable proper 
> > > > userspace block devices.
> > >
> > > Likely slow due do lots of copying.  Also needs a snapshot API.
> >
> > The kernel could use splice.
>
> Still can't make guest memory appear in (A)BUSE process memory without 
> either mmu tricks (vmsplice in reverse) or a copy.  May be workable 
> for an (A)BUSE driver that talks over a network, and thus can splice() 
> its way out.

splice() actually takes offset parameter so it may be possible to treat 
that offset parameter as a file offset.  That would essentially allow 
you to implement a splice() based thread pool where splice() replaces 
preadv/pwritev.

It's not quite linux-aio, but it should take you pretty far.   I think 
the main point is that the problem of allowing block plugins to qemu is 
the same as block plugins for the kernel.  The kernel doesn't provide a 
stable interface (and we probably can't for the same reasons) and it's 
generally discourage from a code quality perspective.

That said, making an external program work well as a block backend is 
identical to making userspace block devices fast.

Regards,

Anthony Liguori