On Wed, Sep 23, 2020 at 12:04 AM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > ... > > Note that if all overlayfs layers are on the same fs and that fs has > > null uuid, then the "disk copy" use case should just work, but I never > > tested that. > > > > So far, there has been no filesystem with null uuid that could be used > > as upper+lower layers (even xfs with option nouuid has non null s_uuid). > > > > Recently, virtiofs was added as a filesystem that could be used for > > upper+lower layers and virtiofs (which is fuse) has null uuid. > > I guess I never paid attention to uuid part of virtiofs. Probably we > are not using index or any of the advanced features of overlayfs yet, > that's why. > I don't expect you should have a problem enabling index because of null uuid when all layers are on the same virtiofs. That setup is allowed. We only ever start checking for null uuid on lower layers that are NOT on the same fs as upper layer. What you are expected to have a problem with is that FUSE support for file handles is "problematic". I found out the hard way that FUSE can decode NFS file handles to completely different object than the encoded object if the encoded inode was evicted from cache and its node id has been reused. Another problem is that FUSE protocol does not have complete support for decoding file handles. FUSE implements decode file handle by LOOKUP(ino, ".") to server, but if server is proxying a local filesystem, there is not enough information to construct an open_by_handle_at() request. I wrote a fuse filesystem whose file handles are persistent and reliable [1], but it is a specialized server that uses knowledge of the local filesystem file handle format and it requires that the local filesystem has a special feature to interpret a file handle with 0 generation as ANY generation (ext4 does that). I think that the proper was to implement reliable persistent file handles in fuse/virtiofs would be to add ENCODE/DECODE to FUSE protocol and allow the server to handle this. Thanks, Amir, [1] https://github.com/amir73il/libfuse/commits/cachegwfs
