On Thu, 24 Jun 2021, J. Bruce Fields wrote: > Is there any hope of solving this problem within btrfs? > > It doesn't seem like it should have been that difficult for it to give > subvolumes separate superblocks and vfsmounts. > > But this has come up before, and I think the answer may have been that > it's just too late to fix. It is never too late to do the right thing! Probably the best approach to fixing this completely on the btrfs side would be to copy the auto-mount approach used in NFS. NFS sees multiple different volumes on the server and transparently creates new vfsmounts, using the automount infrastructure to mount and unmount them. BTRFS effective sees multiple volumes on the block device and could do the same thing. I can only think of one change to the user-space API (other than /proc/mounts contents) that this would cause and I suspect it could be resolved if needed. Currently when you 'stat' the mountpoint of a btrfs subvol you see the root of that subvol. However when you 'stat' the mountpoint of an NFS sub-filesystem (before any access below there) you see the mountpoint (s_dev matches the parent). This is how automounts are expected to work and if btrfs were switched to use automounts for subvols, stating the mountpoint would initially show the mountpoint, not the subvol root. If this were seen to be a problem I doubt it would be hard to add optional functionality to automount so that 'stat' triggers the mount. All we really need is: 1/ someone to write the code 2/ someone to review the code 3/ someone to accept the code How hard can it be :-) NeilBrown
