On Thu, Dec 28, 2023 at 02:53:12PM -0800, Carl E. Thompson wrote: > Hello, there is an issue (bug or missing needed feature) in bcachefs > where it appears to be impossible to mount a bcachefs filesystem which > has the same UUID as an already mounted filesystem. This is necessary > in many forensic and online backup scenarios. For example, in my > personal and professional workflows crash-consistent online backups > are made by mounting thin LVM snapshots. This seems to be impossible > with bcachefs because it lacks a mount option to bypass the unique UID > test and the mount fails with an `Invalid argument` message from mount > on the command line and a `filesystem UUID already open` message in > the kernel log. > > Other filesystems have a mount option specifically to allow this. For > example, XFS has a `nouuid` option. I believe that bcachefs needs a > similar option to be broadly useful, particularly for non-hobbyist use > cases. (If it makes it easier to implement IMO for most uses if the > new mount option to bypass the UUID check implied the `nochanges` > option that would be fine.) > > I am aware that I could just use bcachefs native snapshots instead > (when they are proven to work reliably). However, a large part of the > reason I use LVM is because it gives me a consistent interface for > snapshots (and thus for backups) for all the different filesystems I > use. Retooling to special-case for bcachefs would require a lot of > work and testing and would remove a huge benefit of this approach.
Tricky one, because we're multi device so we have a lot of code that actually needs the UUID for identifying which filesystem a device is in; we probably would want it to imply -o nochanges for it to be remotely safe. Another item for the todo list :)
