On Tue, Sep 12, 2023 at 05:20:42PM +0800, Anand Jain wrote: > > > On 12/09/2023 02:28, David Sterba wrote: > > 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. > > Guliherme, > > Please also add the newly sent patch to the latest misc-next branch: > [PATCH] btrfs: scan forget for no instance of dev > > The test case btrfs/254 fails without it. The mention patch has been folded to the scanning/register patch so you may now use misc-next as a base.
