[Virtio-fs] Live migration support for virtio-fs

[Dr. David Alan Gilbert]

* Vivek Goyal (vgoyal at redhat.com) wrote:
> On Tue, Mar 23, 2021 at 10:29:09AM +0000, Dr. David Alan Gilbert wrote:
> > * Ge, Xiongzi (Xiongzi.Ge at netapp.com) wrote:
> > > Hello Vivek, Dr. Gilbert, and the virtio-fs team,
> > 
> > Hi Xiongzi,
> > 
> > > It seems that virtio-fs does not support live migration. Once a vm
> > > is configured with virtio-fs, live migration cannot be performed even
> > > it is not mounted on the guest.
> > 
> > Right.
> > 
> > > Is there any progress for this?
> > 
> > Max Reitz is looking at some parts of it; in particular storing file
> > handles that can later be reopened.
> > But there are a bunch of other parts we've not looked at yet either
> > (like the dirty page marking around all syscalls).
> > 
> > > If the
> > > shared directory is from a distributed file system like Ceph, would it
> > > be easier than the general case to be implemented?
> > 
> > Maybe; there are some tricky semantics problems; for example, lets
> > imagine that you open the file   'a/b/c'  on the source, and sometime
> > after you open it, 'b' gets renamed to 'd';  when you resume on the
> > destination you need to make sure you know how to get to that file.
> > Depending on the filesystem semantics you might need to make that work
> > even if 'a/b/c' had been deleted but you still had it open.
> 
> Hi Dave,
> 
> I am assuming that with file handles, renaming of file probably is not
> a problem. open_by_handle_at() will still be able to find it.

Right; although again with soemthing like CEPH you might not even need
that if you had a virtiofs daemon that spoke direct to CEPH, you might
be dealing with a CEPH filehandle.

> I think real problem (as you pointed out later) is unliked file which is
> still in use by the virtiofsd.

Dave

> Thanks
> Vivek
-- 
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK