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[Bug 1377142] Re: Bcache doesn't allow full unregistering without rebooting

 

** Description changed:

  If you create a bcache device, you can't reuse all your disk/partitions
  without a reboot.
  
  You can reproduce the case this way:
  
  start a vm with 2 disks (caching must be bigger or equal to the backing
  cf bug :1377130) and an iso of utopic desktop amd64
  
  create the bcache device:
  
-                 make-bcache --writeback --discard -C /dev/sda -B
- /dev/sdb
- 
- 		UUID:			b245150d-cfbe-4f90-836a-343e0e1a4c55
- 		Set UUID:		c990a31a-f531-4231-9603-d40230dc6504
- 		version:		0
- 		nbuckets:		16384
- 		block_size:		1
- 		bucket_size:		1024
- 		nr_in_set:		1
- 		nr_this_dev:		0
- 		first_bucket:		1
- 		UUID:			cc31e0bb-db29-4115-a1b2-e9ff54e5f127
- 		Set UUID:		c990a31a-f531-4231-9603-d40230dc6504
- 		version:		1
- 		block_size:		1
- 		data_offset:		16
- 
- 		******************************
- 		command: 
- 		lsblk
- 
- 		result:
- 		NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
- 		sda         8:0    0    8G  0 disk 
- 		└─bcache0 251:0    0   16G  0 disk 
- 		sdb         8:16   0   16G  0 disk 
- 		└─bcache0 251:0    0   16G  0 disk 
- 		sr0        11:0    1  1,1G  0 rom  /cdrom
- 		loop0       7:0    0  1,1G  1 loop /rofs
- 		******************************
+   make-bcache --writeback --discard -C /dev/sda -B /dev/sdb
+ 
+   UUID:			b245150d-cfbe-4f90-836a-343e0e1a4c55
+   Set UUID:		c990a31a-f531-4231-9603-d40230dc6504
+   version:		0
+   nbuckets:		16384
+   block_size:		1
+   bucket_size:		1024
+   nr_in_set:		1
+   nr_this_dev:		0
+   first_bucket:		1
+   UUID:			cc31e0bb-db29-4115-a1b2-e9ff54e5f127
+   Set UUID:		c990a31a-f531-4231-9603-d40230dc6504
+   version:		1
+   block_size:		1
+   data_offset:		16
+ 
+   ******************************
+   command:
+   lsblk
+ 
+   result:
+   NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
+   sda         8:0    0    8G  0 disk
+   └─bcache0 251:0    0   16G  0 disk
+   sdb         8:16   0   16G  0 disk
+   └─bcache0 251:0    0   16G  0 disk
+   sr0        11:0    1  1,1G  0 rom  /cdrom
+   loop0       7:0    0  1,1G  1 loop /rofs
+   ******************************
  
  All is good
  
- 		lsmod | grep bcache:
- 		bcache                227884  3
+   lsmod | grep bcache:
+   bcache                227884  3
  
  format the bcache device:
  
- 		******************************
- 		command: 
- 		mkfs.ext4 /dev/bcache0
- 
- 		result:
- 		Rejet des blocs de périphérique :    4096/4194302 complété                        
- 		Creating filesystem with 4194302 4k blocks and 1048576 inodes
- 		Filesystem UUID: 587d2249-3eaf-4590-a00d-42939f257e99
- 		Superblocs de secours stockés sur les blocs : 
- 			32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208, 
- 			4096000
- 
- 		Allocation des tables de groupe :   0/128 complété                        
- 		Écriture des tables d'i-noeuds :   0/128 complété                        
- 		Création du journal (32768 blocs) : complété
- 		Écriture des superblocs et de l'information de comptabilité du système de
- 		fichiers :   0/128   2/128 complété
- 
-                 ******************************
+   ******************************
+   command:
+   mkfs.ext4 /dev/bcache0
+ 
+   result:
+   Rejet des blocs de périphérique :    4096/4194302 complété
+   Creating filesystem with 4194302 4k blocks and 1048576 inodes
+   Filesystem UUID: 587d2249-3eaf-4590-a00d-42939f257e99
+   Superblocs de secours stockés sur les blocs :
+    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
+    4096000
+ 
+   Allocation des tables de groupe :   0/128 complété
+   Écriture des tables d'i-noeuds :   0/128 complété
+   Création du journal (32768 blocs) : complété
+   Écriture des superblocs et de l'information de comptabilité du système de
+   fichiers :   0/128   2/128 complété
+ 
+   ******************************
  
  Now mount it:
- 		mount /dev/bcache0 /mnt/
- 		mkdir /mnt/test_dir
- 		touch /mnt/test_file
+   mount /dev/bcache0 /mnt/
+   mkdir /mnt/test_dir
+   touch /mnt/test_file
  
  state of: /sys/fs/bcache/
- 		ls /sys/fs/bcache/
- 
- 		c990a31a-f531-4231-9603-d40230dc6504
- 		register
- 		register_quiet
+   ls /sys/fs/bcache/
+ 
+   c990a31a-f531-4231-9603-d40230dc6504
+   register
+   register_quiet
  
  bcache-super-show /dev/sda
- 		sb.magic		ok
- 		sb.first_sector		8 [match]
- 		sb.csum			E6A8D9AC496B0B04 [match]
- 		sb.version		3 [cache device]
- 
- 		dev.label		(empty)
- 		dev.uuid		b245150d-cfbe-4f90-836a-343e0e1a4c55
- 		dev.sectors_per_block	1
- 		dev.sectors_per_bucket	1024
- 		dev.cache.first_sector	1024
- 		dev.cache.cache_sectors	16776192
- 		dev.cache.total_sectors	16777216
- 		dev.cache.ordered	yes
- 		dev.cache.discard	yes
- 		dev.cache.pos		0
- 		dev.cache.replacement	0 [lru]
- 
- 		cset.uuid		c990a31a-f531-4231-9603-d40230dc6504
- 		******************************
+   sb.magic		ok
+   sb.first_sector		8 [match]
+   sb.csum			E6A8D9AC496B0B04 [match]
+   sb.version		3 [cache device]
+ 
+   dev.label		(empty)
+   dev.uuid		b245150d-cfbe-4f90-836a-343e0e1a4c55
+   dev.sectors_per_block	1
+   dev.sectors_per_bucket	1024
+   dev.cache.first_sector	1024
+   dev.cache.cache_sectors	16776192
+   dev.cache.total_sectors	16777216
+   dev.cache.ordered	yes
+   dev.cache.discard	yes
+   dev.cache.pos		0
+   dev.cache.replacement	0 [lru]
+ 
+   cset.uuid		c990a31a-f531-4231-9603-d40230dc6504
+   ******************************
  
  bcache-super-show -f /dev/sdb
  
- 		sb.magic		ok
- 		sb.first_sector		8 [match]
- 		sb.csum			9600B159F36A67DD [match]
- 		sb.version		1 [backing device]
- 
- 		dev.label		(empty)
- 		dev.uuid		cc31e0bb-db29-4115-a1b2-e9ff54e5f127
- 		dev.sectors_per_block	1
- 		dev.sectors_per_bucket	1024
- 		dev.data.first_sector	16
- 		dev.data.cache_mode	1 [writeback]
- 		dev.data.cache_state	2 [dirty]
- 
- 		cset.uuid		c990a31a-f531-4231-9603-d40230dc6504
- 		******************************
+   sb.magic		ok
+   sb.first_sector		8 [match]
+   sb.csum			9600B159F36A67DD [match]
+   sb.version		1 [backing device]
+ 
+   dev.label		(empty)
+   dev.uuid		cc31e0bb-db29-4115-a1b2-e9ff54e5f127
+   dev.sectors_per_block	1
+   dev.sectors_per_bucket	1024
+   dev.data.first_sector	16
+   dev.data.cache_mode	1 [writeback]
+   dev.data.cache_state	2 [dirty]
+ 
+   cset.uuid		c990a31a-f531-4231-9603-d40230dc6504
+   ******************************
  
  mount:
- 		/dev/bcache0 on /mnt type ext4 (rw)
+   /dev/bcache0 on /mnt type ext4 (rw)
  
  we will unregister the bcache:
- 		echo 1 /sys/fs/bcache/c990a31a-f531-4231-9603-d40230dc6504/unregister
- 
+   echo 1 /sys/fs/bcache/c990a31a-f531-4231-9603-d40230dc6504/unregister
  
  check the content of /sys/fs/bcache
- 		ls /sys/fs/bcache/
- 
- 		register
- 		register_quiet
+   ls /sys/fs/bcache/
+ 
+   register
+   register_quiet
  
  So bcache is unregistered, but status of /dev/sda:
  
- 		bcache-super-show -f /dev/sda
- 		sb.magic		ok
- 		sb.first_sector		8 [match]
- 		sb.csum			E6A8D9AC496B0B04 [match]
- 		sb.version		3 [cache device]
- 
- 		dev.label		(empty)
- 		dev.uuid		b245150d-cfbe-4f90-836a-343e0e1a4c55
- 		dev.sectors_per_block	1
- 		dev.sectors_per_bucket	1024
- 		dev.cache.first_sector	1024
- 		dev.cache.cache_sectors	16776192
- 		dev.cache.total_sectors	16777216
- 		dev.cache.ordered	yes
- 		dev.cache.discard	yes
- 		dev.cache.pos		0
- 		dev.cache.replacement	0 [lru]
- 
-                 cset.uuid
- c990a31a-f531-4231-9603-d40230dc6504
- 
- 
- 		command: 
- 		bcache-super-show -f /dev/sdb
- 
- 		result:
- 		sb.magic		ok
- 		sb.first_sector		8 [match]
- 		sb.csum			D41799F794675FA8 [match]
- 		sb.version		1 [backing device]
- 
- 		dev.label		(empty)
- 		dev.uuid		cc31e0bb-db29-4115-a1b2-e9ff54e5f127
- 		dev.sectors_per_block	1
- 		dev.sectors_per_bucket	1024
- 		dev.data.first_sector	16
- 		dev.data.cache_mode	1 [writeback]
- 		dev.data.cache_state	0 [detached]
- 
- 		cset.uuid		00000000-0000-0000-0000-000000000000
- 		******************************
+   bcache-super-show -f /dev/sda
+   sb.magic		ok
+   sb.first_sector		8 [match]
+   sb.csum			E6A8D9AC496B0B04 [match]
+   sb.version		3 [cache device]
+ 
+   dev.label		(empty)
+   dev.uuid		b245150d-cfbe-4f90-836a-343e0e1a4c55
+   dev.sectors_per_block	1
+   dev.sectors_per_bucket	1024
+   dev.cache.first_sector	1024
+   dev.cache.cache_sectors	16776192
+   dev.cache.total_sectors	16777216
+   dev.cache.ordered	yes
+   dev.cache.discard	yes
+   dev.cache.pos		0
+   dev.cache.replacement	0 [lru]
+ 
+   cset.uuid             c990a31a-f531-4231-9603-d40230dc6504
+ 
+   command:
+   bcache-super-show -f /dev/sdb
+ 
+   result:
+   sb.magic		ok
+   sb.first_sector		8 [match]
+   sb.csum			D41799F794675FA8 [match]
+   sb.version		1 [backing device]
+ 
+   dev.label		(empty)
+   dev.uuid		cc31e0bb-db29-4115-a1b2-e9ff54e5f127
+   dev.sectors_per_block	1
+   dev.sectors_per_bucket	1024
+   dev.data.first_sector	16
+   dev.data.cache_mode	1 [writeback]
+   dev.data.cache_state	0 [detached]
+ 
+   cset.uuid		00000000-0000-0000-0000-000000000000
+   ******************************
  
  Maybe I'm wrong, but I would expect the cahing device to be 000000000ed
  not the backing device, as we may still want to use the data on it.
  
  But data is still accessible on the mount point.
  
- 		So we wipe /dev/sda now:
- 		******************************
- 		command: 
- 		wipefs -a /dev/sda
- 
- 		result:
- 		/dev/sda: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
- 		******************************
+   So we wipe /dev/sda now:
+   ******************************
+   command:
+   wipefs -a /dev/sda
+ 
+   result:
+   /dev/sda: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+   ******************************
  
  mount:
- 		/dev/bcache0 on /mnt type ext4 (rw)
+   /dev/bcache0 on /mnt type ext4 (rw)
  
  data still there:
- 		******************************
- 		command: 
- 		ls /mnt/
- 
- 		result:
- 		lost+found
- 		test_dir
- 		test_file
- 		******************************
- 
- 		******************************
- 		command: 
- 		lsblk
- 
- 		result:
- 		NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
- 		sda         8:0    0    8G  0 disk 
- 		sdb         8:16   0   16G  0 disk 
- 		└─bcache0 251:0    0   16G  0 disk /mnt
- 		sr0        11:0    1  1,1G  0 rom  /cdrom
- 		loop0       7:0    0  1,1G  1 loop /rofs
- 		******************************
+   ******************************
+   command:
+   ls /mnt/
+ 
+   result:
+   lost+found
+   test_dir
+   test_file
+   ******************************
+ 
+   ******************************
+   command:
+   lsblk
+ 
+   result:
+   NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
+   sda         8:0    0    8G  0 disk
+   sdb         8:16   0   16G  0 disk
+   └─bcache0 251:0    0   16G  0 disk /mnt
+   sr0        11:0    1  1,1G  0 rom  /cdrom
+   loop0       7:0    0  1,1G  1 loop /rofs
+   ******************************
  
  Ok now we can backup our data if we need to, next, we will umount:
- 		umount /mnt/
+   umount /mnt/
  
  ok no errors.
  
- 		lsblk:
- 		NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
- 		sda         8:0    0    8G  0 disk 
- 		sdb         8:16   0   16G  0 disk 
- 		└─bcache0 251:0    0   16G  0 disk 
- 		sr0        11:0    1  1,1G  0 rom  /cdrom
- 		loop0       7:0    0  1,1G  1 loop /rofs
- 		******************************
+   lsblk:
+   NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
+   sda         8:0    0    8G  0 disk
+   sdb         8:16   0   16G  0 disk
+   └─bcache0 251:0    0   16G  0 disk
+   sr0        11:0    1  1,1G  0 rom  /cdrom
+   loop0       7:0    0  1,1G  1 loop /rofs
+   ******************************
  
  bcache0 is still there, ok, but how to delete to unregister it
  definitively?
  
-                 ls /sys/fs/bcache/
- 
- 		result:
- 		register
- 		register_quiet
- 		**************
- 
-                 lsmod
- 
- 		result:
- 		Module                  Size  Used by
- 		bcache                227884  1 
+   ls /sys/fs/bcache/
+ 
+   result:
+   register
+   register_quiet
+   **************
+ 
+   lsmod
+ 
+   result:
+   Module                  Size  Used by
+   bcache                227884  1
  
  we can't wipefs -a /dev/sdb, device is in use, so we reboot machine (NOT
  OK)
  
- 		in the vm:
- 		ls /sys/fs/bcache/
- 
- 		result:
- 		register
- 		register_quiet
+   in the vm:
+   ls /sys/fs/bcache/
+ 
+   result:
+   register
+   register_quiet
  
  update packages and reinstall bcache-tools:
  
-                 bcache-super-show -f /dev/sda
- 
- 		result:
- 		Invalid superblock (bad magic)
- 		sb.magic		bad magic
- 
-                 bcache-super-show -f /dev/sdb
- 
- 		result:
- 		sb.magic		ok
- 		sb.first_sector		8 [match]
- 		sb.csum			D41799F794675FA8 [match]
- 		sb.version		1 [backing device]
- 
- 		dev.label		(empty)
- 		dev.uuid		cc31e0bb-db29-4115-a1b2-e9ff54e5f127
- 		dev.sectors_per_block	1
- 		dev.sectors_per_bucket	1024
- 		dev.data.first_sector	16
- 		dev.data.cache_mode	1 [writeback]
- 		dev.data.cache_state	0 [detached]
- 
-                 cset.uuid
- 00000000-0000-0000-0000-000000000000
+   bcache-super-show -f /dev/sda
+ 
+   result:
+   Invalid superblock (bad magic)
+   sb.magic		bad magic
+ 
+   bcache-super-show -f /dev/sdb
+ 
+   result:
+   sb.magic		ok
+   sb.first_sector		8 [match]
+   sb.csum			D41799F794675FA8 [match]
+   sb.version		1 [backing device]
+ 
+   dev.label		(empty)
+   dev.uuid		cc31e0bb-db29-4115-a1b2-e9ff54e5f127
+   dev.sectors_per_block	1
+   dev.sectors_per_bucket	1024
+   dev.data.first_sector	16
+   dev.data.cache_mode	1 [writeback]
+   dev.data.cache_state	0 [detached]
+ 
+   cset.uuid             00000000-0000-0000-0000-000000000000
  
  Now we can wipe /dev/sdb because we rebooted, and bcache don't use the device anymore:
- 		wipefs -a /dev/sdb
- 
- 		result:
- 		/dev/sdb: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
- 
- **********************************************************
- 
- Now I can re-use my device, but I needed a reboot. Maybe we must also
- have better bcache management tools also.
+   wipefs -a /dev/sdb
+ 
+   result:
+   /dev/sdb: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+ 
+   **********************************************************
+ 
+ Now I can re-use my device, but I needed a reboot. Maybe we also must
+ have better bcache management tools.

-- 
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https://bugs.launchpad.net/bugs/1377142

Title:
  Bcache doesn't allow full unregistering without rebooting

Status in “bcache-tools” package in Ubuntu:
  New
Status in “linux” package in Ubuntu:
  Incomplete

Bug description:
  If you create a bcache device, you can't reuse all your
  disk/partitions without a reboot.

  You can reproduce the case this way:

  start a vm with 2 disks (caching must be bigger or equal to the
  backing cf bug :1377130) and an iso of utopic desktop amd64

  create the bcache device:

    make-bcache --writeback --discard -C /dev/sda -B /dev/sdb

    UUID:			b245150d-cfbe-4f90-836a-343e0e1a4c55
    Set UUID:		c990a31a-f531-4231-9603-d40230dc6504
    version:		0
    nbuckets:		16384
    block_size:		1
    bucket_size:		1024
    nr_in_set:		1
    nr_this_dev:		0
    first_bucket:		1
    UUID:			cc31e0bb-db29-4115-a1b2-e9ff54e5f127
    Set UUID:		c990a31a-f531-4231-9603-d40230dc6504
    version:		1
    block_size:		1
    data_offset:		16

    ******************************
    command:
    lsblk

    result:
    NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
    sda         8:0    0    8G  0 disk
    └─bcache0 251:0    0   16G  0 disk
    sdb         8:16   0   16G  0 disk
    └─bcache0 251:0    0   16G  0 disk
    sr0        11:0    1  1,1G  0 rom  /cdrom
    loop0       7:0    0  1,1G  1 loop /rofs
    ******************************

  All is good

    lsmod | grep bcache:
    bcache                227884  3

  format the bcache device:

    ******************************
    command:
    mkfs.ext4 /dev/bcache0

    result:
    Rejet des blocs de périphérique :    4096/4194302 complété
    Creating filesystem with 4194302 4k blocks and 1048576 inodes
    Filesystem UUID: 587d2249-3eaf-4590-a00d-42939f257e99
    Superblocs de secours stockés sur les blocs :
     32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
     4096000

    Allocation des tables de groupe :   0/128 complété
    Écriture des tables d'i-noeuds :   0/128 complété
    Création du journal (32768 blocs) : complété
    Écriture des superblocs et de l'information de comptabilité du système de
    fichiers :   0/128   2/128 complété

    ******************************

  Now mount it:
    mount /dev/bcache0 /mnt/
    mkdir /mnt/test_dir
    touch /mnt/test_file

  state of: /sys/fs/bcache/
    ls /sys/fs/bcache/

    c990a31a-f531-4231-9603-d40230dc6504
    register
    register_quiet

  bcache-super-show /dev/sda
    sb.magic		ok
    sb.first_sector		8 [match]
    sb.csum			E6A8D9AC496B0B04 [match]
    sb.version		3 [cache device]

    dev.label		(empty)
    dev.uuid		b245150d-cfbe-4f90-836a-343e0e1a4c55
    dev.sectors_per_block	1
    dev.sectors_per_bucket	1024
    dev.cache.first_sector	1024
    dev.cache.cache_sectors	16776192
    dev.cache.total_sectors	16777216
    dev.cache.ordered	yes
    dev.cache.discard	yes
    dev.cache.pos		0
    dev.cache.replacement	0 [lru]

    cset.uuid		c990a31a-f531-4231-9603-d40230dc6504
    ******************************

  bcache-super-show -f /dev/sdb

    sb.magic		ok
    sb.first_sector		8 [match]
    sb.csum			9600B159F36A67DD [match]
    sb.version		1 [backing device]

    dev.label		(empty)
    dev.uuid		cc31e0bb-db29-4115-a1b2-e9ff54e5f127
    dev.sectors_per_block	1
    dev.sectors_per_bucket	1024
    dev.data.first_sector	16
    dev.data.cache_mode	1 [writeback]
    dev.data.cache_state	2 [dirty]

    cset.uuid		c990a31a-f531-4231-9603-d40230dc6504
    ******************************

  mount:
    /dev/bcache0 on /mnt type ext4 (rw)

  we will unregister the bcache:
    echo 1 /sys/fs/bcache/c990a31a-f531-4231-9603-d40230dc6504/unregister

  check the content of /sys/fs/bcache
    ls /sys/fs/bcache/

    register
    register_quiet

  So bcache is unregistered, but status of /dev/sda:

    bcache-super-show -f /dev/sda
    sb.magic		ok
    sb.first_sector		8 [match]
    sb.csum			E6A8D9AC496B0B04 [match]
    sb.version		3 [cache device]

    dev.label		(empty)
    dev.uuid		b245150d-cfbe-4f90-836a-343e0e1a4c55
    dev.sectors_per_block	1
    dev.sectors_per_bucket	1024
    dev.cache.first_sector	1024
    dev.cache.cache_sectors	16776192
    dev.cache.total_sectors	16777216
    dev.cache.ordered	yes
    dev.cache.discard	yes
    dev.cache.pos		0
    dev.cache.replacement	0 [lru]

    cset.uuid             c990a31a-f531-4231-9603-d40230dc6504

    command:
    bcache-super-show -f /dev/sdb

    result:
    sb.magic		ok
    sb.first_sector		8 [match]
    sb.csum			D41799F794675FA8 [match]
    sb.version		1 [backing device]

    dev.label		(empty)
    dev.uuid		cc31e0bb-db29-4115-a1b2-e9ff54e5f127
    dev.sectors_per_block	1
    dev.sectors_per_bucket	1024
    dev.data.first_sector	16
    dev.data.cache_mode	1 [writeback]
    dev.data.cache_state	0 [detached]

    cset.uuid		00000000-0000-0000-0000-000000000000
    ******************************

  Maybe I'm wrong, but I would expect the cahing device to be
  000000000ed not the backing device, as we may still want to use the
  data on it.

  But data is still accessible on the mount point.

    So we wipe /dev/sda now:
    ******************************
    command:
    wipefs -a /dev/sda

    result:
    /dev/sda: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
    ******************************

  mount:
    /dev/bcache0 on /mnt type ext4 (rw)

  data still there:
    ******************************
    command:
    ls /mnt/

    result:
    lost+found
    test_dir
    test_file
    ******************************

    ******************************
    command:
    lsblk

    result:
    NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
    sda         8:0    0    8G  0 disk
    sdb         8:16   0   16G  0 disk
    └─bcache0 251:0    0   16G  0 disk /mnt
    sr0        11:0    1  1,1G  0 rom  /cdrom
    loop0       7:0    0  1,1G  1 loop /rofs
    ******************************

  Ok now we can backup our data if we need to, next, we will umount:
    umount /mnt/

  ok no errors.

    lsblk:
    NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
    sda         8:0    0    8G  0 disk
    sdb         8:16   0   16G  0 disk
    └─bcache0 251:0    0   16G  0 disk
    sr0        11:0    1  1,1G  0 rom  /cdrom
    loop0       7:0    0  1,1G  1 loop /rofs
    ******************************

  bcache0 is still there, ok, but how to delete to unregister it
  definitively?

    ls /sys/fs/bcache/

    result:
    register
    register_quiet
    **************

    lsmod

    result:
    Module                  Size  Used by
    bcache                227884  1

  we can't wipefs -a /dev/sdb, device is in use, so we reboot machine
  (NOT OK)

    in the vm:
    ls /sys/fs/bcache/

    result:
    register
    register_quiet

  update packages and reinstall bcache-tools:

    bcache-super-show -f /dev/sda

    result:
    Invalid superblock (bad magic)
    sb.magic		bad magic

    bcache-super-show -f /dev/sdb

    result:
    sb.magic		ok
    sb.first_sector		8 [match]
    sb.csum			D41799F794675FA8 [match]
    sb.version		1 [backing device]

    dev.label		(empty)
    dev.uuid		cc31e0bb-db29-4115-a1b2-e9ff54e5f127
    dev.sectors_per_block	1
    dev.sectors_per_bucket	1024
    dev.data.first_sector	16
    dev.data.cache_mode	1 [writeback]
    dev.data.cache_state	0 [detached]

    cset.uuid             00000000-0000-0000-0000-000000000000

  Now we can wipe /dev/sdb because we rebooted, and bcache don't use the device anymore:
    wipefs -a /dev/sdb

    result:
    /dev/sdb: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81

    **********************************************************

  Now I can re-use my device, but I needed a reboot. Maybe we also must
  have better bcache management tools.

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References