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.
Thanks.
