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* Qixuan Wu (qixuan.wu at linux.alibaba.com) wrote:
> > We're currently experimenting with something a bit different;
> > we've got a setup that uses a modified version of the FUSE protocol
> > running over vhost-user;  it's:
> >    a) Got the filesystem access split out of qemu into a separate daemon
> >        - that's just a modified version of a normal FUSE filesystem daemon
> >        with the nice bit being that since it's a separate process you
> >        can do whatever isolation on it you want.
> >    b) But the latency is low because vhost-user means the daemon can read
> >       the request queue straight out of the guest memory
> >    c) We've got a setup with DAX so that the files are mapped straight
> >       into guest address space, so the overhead is very low for large
> >       files.
> That's so cool. I guess it will not use virtio. And this way maybe the new
> para-virtualization method, it's specific about the file system for the data
> shared between guest and host.

It does use virtio!  It's basically just the existing FUSE protocol
carried over virtio; it's got some tweaks to allow the direct mappings
and to deal with some difference sin the setup.
It uses the existing vhost-user implementation of virtio (just like
vhost-user for network does virtio for dpdk).

> >    d) We've got a caching scheme for metadata, which again removes a lot
> >       of latency.
> >    e) We've got some patches to use it in KATA; I can start a basic KATA
> >       guest with it.
> > 
> > This is the first public mention of it because I didn't want you waiting
> > for a reply; but our code is still rather messy and experimental; give
> > us a few weeks and as soon as it survives some smoke tests we'll make
> > the code public.
> > 
> > Because we're reusing both FUSE and vhost-user the kernel changes are
> > quite small, as are the qemu changes.
> > 
> > I realise that's not much detail yet; we're starting to write some of it
> > up; feel free to ask any specifics.
> > 
> 
> Thank for the replay. Seems that the file data are mmapping direct, but the
> control plane, like metadata are still using some other simple protocal,
> maybe new protocal ?

The control plane again is basically just the existing FUSE protocol;
but we've got a shared mmap'd region for a fast lookup for some of the
metadata.

> Because 9p and nfs are very complex, they are not
> developed for the file sharing between guest and host. I always hope there
> is a simple file sharing protocal. I am very looking forward for the code.
> :-).

Glad you like the sound of it;  we'll try and get it out ASAP.

Dave

> 
> Thanks & Regards
> Qixuan Wu.
--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK