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>
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