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Given numerous disk images with the same metadata_uuid but >>> different fsid, there's no clear way a device can be correctly assembled >>> with the proper partners when the CHANGING_FSID_V2 flag is set. So, the >>> disk could be assembled incorrectly, as in the example below: >>> >>> Before this patch: >>> >>> Consider the following two filesystems: >>>     /dev/loop[2-3] are raw copies of /dev/loop[0-1] and the btrsftune -m >>> operation fails. >>> >>> In this scenario, as the /dev/loop0's fsid change is interrupted, and >>> the >>> CHANGING_FSID_V2 flag is set as shown below. >>> >>>    $ p="device|devid|^metadata_uuid|^fsid|^incom|^generation|^flags" >>> >>>    $ btrfs inspect dump-super /dev/loop0 | egrep '$p' >>>    superblock: bytenr=65536, device=/dev/loop0 >>>    flags            0x1000000001 >>>    fsid            7d4b4b93-2b27-4432-b4e4-4be1fbccbd45 >>>    metadata_uuid        bb040a9f-233a-4de2-ad84-49aa5a28059b >>>    generation        9 >>>    num_devices        2 >>>    incompat_flags    0x741 >>>    dev_item.devid    1 >>> >>>    $ btrfs inspect dump-super /dev/loop1 | egrep '$p' >>>    superblock: bytenr=65536, device=/dev/loop1 >>>    flags            0x1 >>>    fsid            11d2af4d-1b71-45a9-83f6-f2100766939d >>>    metadata_uuid        bb040a9f-233a-4de2-ad84-49aa5a28059b >>>    generation        10 >>>    num_devices        2 >>>    incompat_flags    0x741 >>>    dev_item.devid    2 >>> >>>    $ btrfs inspect dump-super /dev/loop2 | egrep '$p' >>>    superblock: bytenr=65536, device=/dev/loop2 >>>    flags            0x1 >>>    fsid            7d4b4b93-2b27-4432-b4e4-4be1fbccbd45 >>>    metadata_uuid        bb040a9f-233a-4de2-ad84-49aa5a28059b >>>    generation        8 >>>    num_devices        2 >>>    incompat_flags    0x741 >>>    dev_item.devid    1 >>> >>>    $ btrfs inspect dump-super /dev/loop3 | egrep '$p' >>>    superblock: bytenr=65536, device=/dev/loop3 >>>    flags            0x1 >>>    fsid            7d4b4b93-2b27-4432-b4e4-4be1fbccbd45 >>>    metadata_uuid        bb040a9f-233a-4de2-ad84-49aa5a28059b >>>    generation        8 >>>    num_devices        2 >>>    incompat_flags    0x741 >>>    dev_item.devid    2 >>> >>> >>> It is normal that some devices aren't instantly discovered during >>> system boot or iSCSI discovery. The controlled scan below demonstrates >>> this. >>> >>>    $ btrfs device scan --forget >>>    $ btrfs device scan /dev/loop0 >>>    Scanning for btrfs filesystems on '/dev/loop0' >>>    $ mount /dev/loop3 /btrfs >>>    $ btrfs filesystem show -m >>>    Label: none  uuid: 7d4b4b93-2b27-4432-b4e4-4be1fbccbd45 >>>     Total devices 2 FS bytes used 144.00KiB >>>     devid    1 size 300.00MiB used 48.00MiB path /dev/loop0 >>>     devid    2 size 300.00MiB used 40.00MiB path /dev/loop3 >>> >>> /dev/loop0 and /dev/loop3 are incorrectly partnered. >>> >>> This kernel patch removes functions and code connected to the >>> CHANGING_FSID_V2 flag. >> >> I didn't have a closer look but it seems you're removing all the logic >> to make the metadata uuid robust and usable in case of interrupted >> conversion, while finding another case where it does not work as you >> expect. With this it would be change in behaviour, we need to check >> the original use case. IIRC as the metadata uuid change is lightweight >> we want to try harder to deal with the easy errors instead of rejecting >> the filesystem mount. > > Robust indeed. Silently assembling wrong devices-data loss risk? > Failing to assemble is still safe. > > I think it is better to introduce a sub-command to clone btrfs > filesystem with a new device-uuid and same fsid (as it looks like > same fsid has some use case). > > Thanks, Anand Oh, my memory comes back, the original design for the two stage commitment is to avoid split brain cases when one device is committed with the new flag, while the remaining one doesn't. With the extra stage, even if at stage 1 or 2 the transaction is interrupted and only one device got the new flag, it can help us to locate the stage and recover. Thanks, Qu