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

[Stefan Hajnoczi]

On Tue, Mar 23, 2021 at 01:35:58AM +0000, Ge, Xiongzi wrote:
> 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. Is there any progress for this? If the shared directory is from a distributed file system like Ceph, would it be easier than the general case to be implemented? 

The virtio-fs device holds a bunch of state, like the ino_map, dirp_map,
and the fd_map in the virtiofsd C implementation. That is the FUSE
session state that needs to be migrated in order to resume seamlessly
(without unmounting and mounting again).

If the backend is a distributed file system then it my have APIs that
make migration easier. If it's possible to re-attach to open files from
another host then that is perfect. But on the flipside if there are no
APIs for doing that then it might be impossible to reliably live
migration because some state cannot be transferred between hosts.

Max Reitz is laying the foundation for live migration by working on
open_by_handle_at(2) support. This will probably be needed in order to
transfer open files from one host to another.

It should be possible to enable live migration when the filesystem is
not mounted. That might be a good first step to enabling live migration.

Stefan
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