On Wed, Aug 30, 2023 at 09:12:34PM -0300, Guilherme G. Piccoli wrote: > Btrfs doesn't currently support to mount 2 different devices holding the > same filesystem - the fsid is exposed as a unique identifier by the > driver. This case is supported though in some other common filesystems, > like ext4. > > Supporting the same-fsid mounts has the advantage of allowing btrfs to > be used in A/B partitioned devices, like mobile phones or the Steam Deck > for example. Without this support, it's not safe for users to keep the > same "image version" in both A and B partitions, a setup that is quite > common for development, for example. Also, as a big bonus, it allows fs > integrity check based on block devices for RO devices (whereas currently > it is required that both have different fsid, breaking the block device > hash comparison). > > Such same-fsid mounting is hereby added through the usage of the > filesystem feature "single-dev" - when such feature is used, btrfs > generates a random fsid for the filesystem and leverages the long-term > present metadata_uuid infrastructure to enable the usage of this > secondary virtual fsid, effectively requiring few non-invasive changes > to the code and no new potential corner cases. > > In order to prevent more code complexity and corner cases, given > the nature of this mechanism (single devices), the single-dev feature > is not allowed when the metadata_uuid flag is already present on the > fs, or if the device is on fsid-change state. Device removal/replace > is also disabled for devices presenting the single-dev feature. > > Suggested-by: John Schoenick <johns@xxxxxxxxxxxxxxxxx> > Suggested-by: Qu Wenruo <wqu@xxxxxxxx> > Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx> I'd like to pick this as a feature for 6.7, it's extending code we already have for metadata_uuid so this is a low risk feature. The only problem I see for now is the name, using the word 'single'. We have single as a block group profile name and a filesystem can exist on a single device too, this is would be confusing when referring to it. Single-dev can be a working name but for a final release we should really try to pick something more unique. I don't have a suggestion for now. The plan for now is that I'll add the patch to a topic branch and add it to for-next so it could be tested but there might be some updates still needed. Either as changes to this patch or as separate patches, that depends.
