When you have btrfs mount where there is a both a subvolume of that mount, and the source of a bind-mount under that mount, df will report confusing results. It will show results for the bind-mount instead of the main btrfs mount when you pass the subvoulme on the command line. It may (depending on the system) show both the bind-mount and main mount in bare df results.
I've attached a script with the minimal repro instructions. It needs to be run as root since it (temporarily) mounts filesystems under your tmpdir. I tested on two machines: Fedora 20 running kernel 3.14.4-200.fc20.x86_64, against both the system df 8.21 and df 8.22 built from source. Centos 6.4 running a 3.10.39 variant, with significant backports, notably including most btrfs changes from mainline. Testing against the system df 8.4, and df 8.22 again built from source. On the fedora machine, /etc/mtab is a symlink to /proc/self/mounts. We see output that refers to the correct block device in all cases. However, we see a report for the bind-mount destination instead of the main btrfs mount when asking specifically for the subvolume path, and see results for both the main btrfs filesystem and the bind-mount when we run a bare df. Transcript of Fedora output: [root@caitsidhe dschleimer]# ./repro_instructions.sh 1024+0 records in 1024+0 records out 10485760 bytes (10 MB) copied, 0.00963354 s, 1.1 GB/s SMALL VOLUME: forcing mixed metadata/data groups WARNING! - Btrfs v3.12 IS EXPERIMENTAL WARNING! - see http://btrfs.wiki.kernel.org before using Turning ON incompat feature 'mixed-bg': mixed data and metadata block groups Turning ON incompat feature 'extref': increased hardlink limit per file to 65536 Created a data/metadata chunk of size 1048576 fs created label (null) on block nodesize 4096 leafsize 4096 sectorsize 4096 size 10.00MiB Btrfs v3.12 Create subvolume 'filesystem/subvolume' Begin unexpected output Filesystem 1K-blocks Used Available Use% Mounted on /dev/loop0 10240 36 6112 1% /tmp/tmp.biUryZcwD9/binddest Expected report for the main btrfs mount, not the bind-mount /dev/loop0 btrfs 10240 36 6112 1% /tmp/tmp.biUryZcwD9/filesystem /dev/loop0 btrfs 10240 36 6112 1% /tmp/tmp.biUryZcwD9/binddest Expected only one btrfs filesystem /home/dschleimer [root@caitsidhe dschleimer]# PATH=/home/dschleimer/Downloads/coreutils-8.22/src/:$PATH ./repro_instructions.sh 1024+0 records in 1024+0 records out 10485760 bytes (10 MB) copied, 0.012752 s, 822 MB/s SMALL VOLUME: forcing mixed metadata/data groups WARNING! - Btrfs v3.12 IS EXPERIMENTAL WARNING! - see http://btrfs.wiki.kernel.org before using Turning ON incompat feature 'mixed-bg': mixed data and metadata block groups Turning ON incompat feature 'extref': increased hardlink limit per file to 65536 Created a data/metadata chunk of size 1048576 fs created label (null) on block nodesize 4096 leafsize 4096 sectorsize 4096 size 10.00MiB Btrfs v3.12 Create subvolume 'filesystem/subvolume' Begin unexpected output Filesystem 1K-blocks Used Available Use% Mounted on /dev/loop0 10240 36 6112 1% /tmp/tmp.TEBR2D9Vta/binddest Expected report for the main btrfs mount, not the bind-mount /dev/loop0 btrfs 10240 36 6112 1% /tmp/tmp.TEBR2D9Vta/filesystem /dev/loop0 btrfs 10240 36 6112 1% /tmp/tmp.TEBR2D9Vta/binddest Expected only one btrfs filesystem /home/dschleimer On Centos, /etc/mtab is a plain file. When asking specifically for the subvolume, we see a report for the bind-mount dest which lists the bind-mount source as the filesystem. When we run a bare df, we see only one record with the file backing the loopback device as the filesystem, but the bind-mount as the mount-point. Centos transcript: [08:03:09 Tue Jun 03 2014] root@devbig100.prn2 /home/dschleimer dschleimer 282 $ ./repro_instructions.sh 1024+0 records in 1024+0 records out 10485760 bytes (10 MB) copied, 0.0101568 s, 1.0 GB/s WARNING! - Btrfs v0.20-rc1-358-g194aa4a IS EXPERIMENTAL WARNING! - see http://btrfs.wiki.kernel.org before using SMALL VOLUME: forcing mixed metadata/data groups Created a data/metadata chunk of size 1048576 fs created label (null) on block nodesize 4096 leafsize 4096 sectorsize 4096 size 10.00MB Btrfs v0.20-rc1-358-g194aa4a Create subvolume 'filesystem/subvolume' Begin unexpected output Filesystem 1K-blocks Used Available Use% Mounted on /tmp/tmp.V4FNWcBXke/filesystem/subvolume/bindsource 10240 36 6112 1% /tmp/tmp.V4FNWcBXke/binddest Expected report for the main btrfs mount, not the bind-mount df: `/mnt/gvfs': Function not implemented /tmp/tmp.V4FNWcBXke/block btrfs 10240 36 6112 1% /tmp/tmp.V4FNWcBXke/filesystem Expected only one btrfs filesystem /home/dschleimer [08:09:39 Tue Jun 03 2014] root@devbig100.prn2 /home/dschleimer dschleimer 282 $ PATH=/data/users/dschleimer/coreutils-8.22/src/:$PATH ./repro_instructions.sh 1024+0 records in 1024+0 records out 10485760 bytes (10 MB) copied, 0.0116229 s, 902 MB/s WARNING! - Btrfs v0.20-rc1-358-g194aa4a IS EXPERIMENTAL WARNING! - see http://btrfs.wiki.kernel.org before using SMALL VOLUME: forcing mixed metadata/data groups Created a data/metadata chunk of size 1048576 fs created label (null) on block nodesize 4096 leafsize 4096 sectorsize 4096 size 10.00MB Btrfs v0.20-rc1-358-g194aa4a Create subvolume 'filesystem/subvolume' Begin unexpected output Filesystem 1K-blocks Used Available Use% Mounted on /tmp/tmp.o7w6NcN178/filesystem/subvolume/bindsource 10240 36 6112 1% /tmp/tmp.o7w6NcN178/binddest Expected report for the main btrfs mount, not the bind-mount /tmp/tmp.o7w6NcN178/block btrfs 10240 36 6112 1% /tmp/tmp.o7w6NcN178/filesystem Expected only one btrfs filesystem /home/dschleimer
repro_instructions.sh
Description: repro_instructions.sh
