On Thu, May 04, 2023 at 02:07:06PM -0300, Guilherme G. Piccoli wrote: > Hi folks, this is an attempt of supporting same fsid mounting on btrfs. > Currently, we cannot reliably mount same fsid filesystems even one at > a time in btrfs, but if users want to mount them at the same time, it's > pretty much impossible. Other filesystems like ext4 are capable of that. > > The goal is to allow systems with A/B partitioning scheme (like the > Steam Deck console or various mobile devices) to be able to hold > the same filesystem image in both partitions; it also allows to have > block device level check for filesystem integrity - this is used in the > Steam Deck image installation, to check if the current read-only image > is pristine. A bit more details are provided in the following ML thread: > > https://lore.kernel.org/linux-btrfs/c702fe27-8da9-505b-6e27-713edacf723a@xxxxxxxxxx/ > > The mechanism used to achieve it is based in the metadata_uuid feature, > leveraging such code infrastructure for that. The patches are based on > kernel 6.3 and were tested both in a virtual machine as well as in the > Steam Deck. Comments, suggestions and overall feedback is greatly > appreciated - thanks in advance! So how does this work if someone needs to mount 3 copies of the same filesystem at the same time? On XFS, we have the "nouuid" mount option which skips the duplicate UUID checking done at mount time so that multiple snapshots or images of the same filesystem can be mounted at the same time. This means we don't get the same filesystem mounted by accident, but also allows all the cases we know about where multiple versions of the filesystem need to be mounted at the same time. I know, fs UUIDs are used differently in btrfs vs XFS, but it would be nice for users if filesystems shared the same interfaces for doing the same sort of management operations... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
