On Mon, Aug 13, 2018 at 9:35 AM, Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx> wrote: >> If the same block device is visible, with rw access, in two different >> containers, I don't see any anything good can happen. Sure, with the > > At the raw level there are lots of use cases involving high performance > data capture, media streaming and the like. > > At the file system layer you can use GFS2 for example. Ugh. I even thought of this case, and I should have been a bit more precise: I would consider the GFS2 case to be essentially equivalent to the NFS case. I think we can probably divide all the filesystems into three or four types: pseudo file systems: Multiple instantiations of the same fs driver pointing at the same backing store give separate filesystems. (Same backing store includes the case where there isn't any backing store.) tmpfs is an example. This isn't particularly interesting. network-like file systems: Multiple instantiations of the same fs driver pointing at the same backing store are expected. This includes NFS, GFS2, AFS, CIFS, etc. This is only really interesting to the extent that, if the fs driver internally wants to share state between multiple instantiations, it should be smart enough to make sure the options are compatible or that it can otherwise handle mismatched options correctly. NFS does this right. non-network-like filesystems: There are complicated ones like btrfs and ZFS and simple ones like ext4. In either case, multiple totally separate instantiations of the driver sharing the backing store will lead to corruption. In cases like ext4, we seem to support it for legacy reasons, because we're afraid that there are scripts that try to mount the same block device more than once, and I think the new API has no need to support this. In cases like btrfs, we also seem to support multiple user requests for "mounts" with the same underlying block devices because we need it for full functionality. But I think this is because our API is wrong. Are there cases I'm missing? It sounds like the API could be improved to fully model the last case, and everything will work nicely.
