why does fuser gives no result for a mounted BTRFS filesystem, but shows a lot pr processes if that fs is unmounted

[Toralf FÃrster]

I'm faced with a problem to /sbin/btrfsck a filesystem due to "Device or resource busy". fuser does not give reasonable help. Moreover I do wonder about the output of fuser when the filesystem is unmounted (pls see below):

# /sbin/btrfsck /dev/sdb2
Opening filesystem to check...
ERROR: cannot open device '/dev/sda6': Device or resource busy
ERROR: cannot open file system

# /sbin/btrfsck /dev/sda6
Opening filesystem to check...
ERROR: cannot open device '/dev/sdb2': Device or resource busy
ERROR: cannot open file system

# mount /home/tinderbox/img2

#  btrfs filesystem df /home/tinderbox/img2
Data, RAID0: total=1.39TiB, used=1.39TiB
System, RAID1: total=8.00MiB, used=128.00KiB
Metadata, RAID1: total=58.00GiB, used=57.17GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

#  btrfs filesystem usage /home/tinderbox/img2
Overall:
    Device size:                   1.66TiB
    Device allocated:              1.50TiB
    Device unallocated:          161.98GiB
    Device missing:                  0.00B
    Used:                          1.50TiB
    Free (estimated):            163.76GiB      (min: 82.77GiB)
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)

Data,RAID0: Size:1.39TiB, Used:1.39TiB (99.87%)
   /dev/sda6     711.00GiB
   /dev/sdb2     711.00GiB

Metadata,RAID1: Size:58.00GiB, Used:57.17GiB (98.56%)
   /dev/sda6      58.00GiB
   /dev/sdb2      58.00GiB

System,RAID1: Size:8.00MiB, Used:128.00KiB (1.56%)
   /dev/sda6       8.00MiB
   /dev/sdb2       8.00MiB

Unallocated:
   /dev/sda6      80.99GiB
   /dev/sdb2      80.99GiB

# fuser -m /home/tinderbox/img2

# umount /home/tinderbox/img2

# fuser -m /home/tinderbox/img2
/home/tinderbox/img2:     1rce     2rc     3rc     4rc     6rc     8rc     9rc    10rc    11rc    12rc    13rc    14rc    15rc    17rc    18rc    19rc    20rc    22rc    23rc    24rc    25rc    27rc    28rc    29rc    30rc    32rc    33rc    34rc    35rc    37rc    38rc    39rc    40rc    42rc    43rc    44rc    45rc    47rc    48rc    49rc    50rc    52rc    53rc    54rc    55rc    57rc    58rc    59rc    60rc    62rc    63rc    64rc    65rc    67rc    69rc    72rc    73rc    74rc    75rc    87rc   106rc   107rc   109rc   112rc   114rc   115rc   123rc   124rc   125rc   126rc   127rc   128rc   129rc   130rc   131rc   132rc   133rc   134rc   135rc   142rc   169rc   170rc   174rc   733rc   753rc   788rc   914rc   915rc   916rc   917rc   924rc   960rc   995rc  1102rc  1104rc  1105rc  1106rc  1107rc  1108rc  1109rc  1110rc  1111rc  1112rc  1113rc  1114rc  1115rc  1116rc  1117rc  1118rc  1119rc  1120rc  1121rc  1122rc  1140rc  1143rc  1144rc  1145rc  1147rc  1148rc  1149rc  1150rc  1151rc  1152rc  1153rc  1154rc  1155rc  1156rc  1157rc  1158rc  1159rc  1160rc  1161rc  1162rc  1414rce  1416rce  1419rce  1588rc  1990rc  2324rce  2327rce  2567rc  2571rc  2613rce  2619rce  2635rce  2655  2657rc  2661  2739rc  2746rc  2778rce  2784rce  2832  2836  3719rc  3784rce  3787rce  3827  3833  4011rce  4012rce  4013rce  4014rce  4015rce  4019rc  4024rce  4398rc  4695rc  4848rc  4849rc  4940rc  5221rce  5547rce  5552rce  5569rce  5654rce  5657rce  5658re  5780rc  5883rc  5952rc  5979rce  5980rce  6192rc  6757rc  6761rc  6928rc  7802rc  8121rc  8396rc  8446rc  8817rc  8927rc  9021rc  9024rc  9053rce  9122rc  9223rc  9360rc  9447rc  9987rc 10366rc 10624rc 10633rc 10949rc 10973rc 11190rc 11237rc 12008rc 12231rc 12273rc 12439rc 12539rc 12541rc 12586rce 12643rc 13174rc 13186rc 13688rc 14414rc 14673rc 14716rce 14815rce 15034rce 15038rce 15059 15060 15073rc 15115rc 15363rc 15376rce 15407rce 15694rce 15737rce 15779rc 15797rce 15809rce 15840rce 16138rce 16141rce 16142rce 16179rc 16428rc 16675rc 17766rc 17826rc 18367rce 18638rc 19842rce 20673rc 21303rc 22148rc 22153rc 22226rce 22485rc 22787rc 22996rc 23104rc 23105rc 23686rce 23835rce 23902rc 24127rc 24130rc 24616rce 24617rce 25093rce 25472rc 25473rc 25474rc 25475rc 25477rc 25478rc 25479rc 25480rc 25481rc 25482rc 25483rc 25484rc 25485rc 25486rc 25488rc 25489rc 25490rc 25844rc 25877rc 25979rc 26521rc 27035rc 27096rc 27183rc 27184rc 27188rc 27383rc 27384rc 27385rc 27391rce 27392re 28157rc 28264rc 28510rc 29035rc 29167rc 29426rc 29959rc 30122rc 30302rc 30474rce 30475rce 30476rce 30477rce 30479rce 30480rce 30481rce 30482rce 30483rce 30484rce 30485rce 30486rce 30488rce 30489rce 30490rce 30491rce 30492rce 30493rce 30494rce 30495rce 30496rce 30497rce 30498rce 30499rce 30500rce 30501rce 30636rce 30645rce 30711 30748 30870rce 30872rce 30897 30919 31005rce 31007rce 31029 31034 31512rc 32184rce 32186rce 32207 32209 32738rce


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Toralf