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

From: Toralf FÃrster
Date: Sun Apr 26 2020 - 14:09:48 EST


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