Comment # 2 on bug 964969 from Ulrich Windl

Explanation first:
What I have is a standard btrfs root layout, so the filesystem in question may
be /.  However / wasn't written, when the problem occurred, but /var/lib/dp/big
(another btrfs filesystem without subvolumes). For completeness I give you the
stats for all btrfs filesystems. Note what happens with /test/raid1!

s1:~ # btrfs fi df /
Data, single: total=13.94GiB, used=8.79GiB
System, DUP: total=32.00MiB, used=16.00KiB
Metadata, DUP: total=1.00GiB, used=799.77MiB
GlobalReserve, single: total=224.00MiB, used=0.00B

So this looks fine, right?

s1:~ # btrfs fi df /var/lib/dp/big
Data, single: total=14.00GiB, used=13.72GiB
System, DUP: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=1.00GiB, used=24.09MiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=16.00MiB, used=0.00B
s1:~ # btrfs fi df /var/lib/dp/lib
Data, single: total=5.01GiB, used=2.25GiB
System, DUP: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=1.00GiB, used=3.31MiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=16.00MiB, used=0.00B
s1:~ # btrfs fi df /var/lib/dp/lib2
Data, single: total=3.01GiB, used=2.26GiB
System, DUP: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=1.00GiB, used=2.88MiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=16.00MiB, used=0.00B
s1:~ # btrfs fi df /test/raid1
ERROR: couldn't get space info - Inappropriate ioctl for device
ERROR: get_df failed Inappropriate ioctl for device
s1:~ # btrfs fi df /test/raid2
Data, single: total=5.01GiB, used=4.41GiB
System, DUP: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=1.00GiB, used=7.25MiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=16.00MiB, used=0.00B
s1:~ # btrfs fi df /test/raid3
Data, single: total=5.01GiB, used=4.41GiB
System, DUP: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=1.00GiB, used=7.25MiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=16.00MiB, used=0.00B

s1:~ # mount /test/raid1
mount: /dev/mapper/FC70R10-lvol1 is already mounted or /test/raid1 busy
       /dev/mapper/FC70R10-lvol1 is already mounted on /test/raid1
s1:~ # btrfs fi df /test/raid1
ERROR: couldn't get space info - Inappropriate ioctl for device
ERROR: get_df failed Inappropriate ioctl for device
s1:~ # umount /test/raid1
s1:~ # mount /test/raid1
s1:~ # btrfs fi df /test/raid1
ERROR: couldn't get space info - Inappropriate ioctl for device
ERROR: get_df failed Inappropriate ioctl for device

#related dmesg output:
[75548.420306] XFS (dm-5): Unmounting Filesystem
[75551.146007] XFS (dm-5): Mounting V5 Filesystem
[75551.290050] XFS (dm-5): Ending clean mount

s1:~ # df -h / /var/lib/dp/* /test/raid*
Filesystem                   Size  Used Avail Use% Mounted on
/dev/mapper/sys-root          16G   11G  5.2G  68% /
/dev/mapper/play-big         266G   14G  250G   6% /var/lib/dp/big
/dev/mapper/play-lib         100G  2.3G   96G   3% /var/lib/dp/lib
/dev/mapper/play-lib2         48G  2.3G   44G   5% /var/lib/dp/lib2
/dev/mapper/FC70R10-lvol1    274G  4.7G  269G   2% /test/raid1
/dev/mapper/FC300R10-lvol1   3.2T  4.5G  3.2T   1% /test/raid2
/dev/mapper/FC300FR10-lvol1  3.3T  4.5G  3.3T   1% /test/raid3