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The BTRFS_SUPER_FLAG_CHANGING_FSID_V2 flag indicates a transient state where the device in the userspace btrfstune -m|-M operation failed to complete changing the fsid. This flag makes the kernel to automatically determine the other partner devices to which a given device can be associated, based on the fsid, metadata_uuid and generation values. btrfstune -m|M feature is especially useful in virtual cloud setups, where compute instances (disk images) are quickly copied, fsid changed, and launched. 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. With this patch, now devices with the CHANGING_FSID_V2 flag are rejected. And its partner will fail to mount with the extra -o degraded option. The check is removed from open_ctree(), devices are rejected during scanning which in turn fails the mount. Signed-off-by: Anand Jain <anand.jain@oracle.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.