Re: [PATCH V3 2/2] btrfs: Introduce the single-dev feature

[Anand Jain <anand.jain@oracle.com>]

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@valvesoftware.com>
>> Suggested-by: Qu Wenruo <wqu@suse.com>
>> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> 
> I've added Anand's patch
> https://lore.kernel.org/linux-btrfs/de8d71b1b08f2c6ce75e3c45ee801659ecd4dc43.1694164368.git.anand.jain@oracle.com/
> 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.