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've added Anand's patch https://lore.kernel.org/linux-btrfs/de8d71b1b08f2c6ce75e3c45ee801659ecd4dc43.1694164368.git.anand.jain@xxxxxxxxxx/ to misc-next that implements subset of your patch, namely extending btrfs_scan_one_device() with the 'mounting' parameter. I haven't looked if the semantics is the same so I let you take a look. As there were more comments to V3, please fix that and resend. Thanks.
