Seems to be some inconsistency with btrfs check --clear-space-cache v1.
It only worked after clearing v2 space cache. See below result.
Background. Since some time I was running with space_cache mount option.
I recently changed to space_cache=v2 and used "clear_cache" mount option
to clear out the old v1 cache. From reading this mailings list I found
that I should really use btrfs check --clear-space-cache v1 to truly
clear the old version space cache.
btrfs-progs-4.13
kernel-4.13
# btrfs check --clear-space-cache v1 /dev/sdb1
couldn't open RDWR because of unsupported option features (3).
ERROR: cannot open file system
# btrfs check /dev/sdb1
Checking filesystem on /dev/sdb1
UUID: e0ea1dc2-948a-488c-ad46-98236939b373
checking extents
checking free space tree
cache and super generation don't match, space cache will be invalidated
checking fs roots
checking csums
checking root refs
found 305286168601 bytes used, no error found
total csum bytes: 246501768
total tree bytes: 415334400
total fs tree bytes: 87130112
total extent tree bytes: 45481984
btree space waste bytes: 65894664
file data blocks allocated: 5691562237952
referenced 341901901824
# btrfs check --clear-space-cache v1 /dev/sdb1
couldn't open RDWR because of unsupported option features (3).
ERROR: cannot open file system
# btrfs check --clear-space-cache v2 /dev/sdb1
Checking filesystem on /dev/sdb1
UUID: e0ea1dc2-948a-488c-ad46-98236939b373
Clear free space cache v2
free space cache v2 cleared
# btrfs check --clear-space-cache v2 /dev/sdb1
Checking filesystem on /dev/sdb1
UUID: e0ea1dc2-948a-488c-ad46-98236939b373
no free space cache v2 to clear
# btrfs check --clear-space-cache v1 /dev/sdb1
Checking filesystem on /dev/sdb1
UUID: e0ea1dc2-948a-488c-ad46-98236939b373
Clearing free space cache
Free space cache cleared
# btrfs check /dev/sdb1
Checking filesystem on /dev/sdb1
UUID: e0ea1dc2-948a-488c-ad46-98236939b373
checking extents
checking free space tree
checking fs roots
checking csums
checking root refs
found 305220087844 bytes used, no error found
total csum bytes: 246501768
total tree bytes: 415236096
total fs tree bytes: 87130112
total extent tree bytes: 45481984
btree space waste bytes: 65905890
file data blocks allocated: 5691495915520
referenced 341835579392
Now, why do I not get "no free space cache v1 to clear" when running it
several times more?
# btrfs check --clear-space-cache v1 /dev/sdb1
Checking filesystem on /dev/sdb1
UUID: e0ea1dc2-948a-488c-ad46-98236939b373
Clearing free space cache
The exact same happen on my other volumes, both single devices and RAID1.
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