I know that "df -h" is not to be trusted with btrfs, but some userspace
tools (i.e. apt, monitoring checks) rely on it.
Sometimes, "df -h" will report 100% disk usage on a btrfs filesystem,
while there is still space available and writes are possible.
Linux 5.4.1, LXD containers located in /data:
# df -h
/dev/nvme1n1 1000G 571G 0 100% /data
# btrfs fi usage /data
Overall:
Device size: 1000.00GiB
Device allocated: 580.02GiB
Device unallocated: 419.98GiB
Device missing: 0.00B
Used: 570.06GiB
Free (estimated): 429.45GiB (min: 429.45GiB)
Data ratio: 1.00
Metadata ratio: 1.00
Global reserve: 512.00MiB (used: 0.00B)
Data,single: Size:578.01GiB, Used:568.54GiB
/dev/nvme1n1 578.01GiB
Metadata,single: Size:2.01GiB, Used:1.53GiB
/dev/nvme1n1 2.01GiB
System,single: Size:4.00MiB, Used:80.00KiB
/dev/nvme1n1 4.00MiB
Unallocated:
/dev/nvme1n1 419.98GiB
From container point of view, it looks like:
container# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/nvme1n1 1000G 571G 0 100% /
Let's see if we can still write? Yes we can:
container# dd if=/dev/urandom of=/bigfile.img bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 7.28306 s, 147 MB/s
container# sync
container# rm /bigfile.img
Let's try to use apt - unfortunately it says that there is not enough
space in /var/cache/apt/archives/:
container# apt install lzop
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
liblzo2-2
The following NEW packages will be installed:
liblzo2-2 lzop
0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 88.5 kB of archives.
After this operation, 282 kB of additional disk space will be used.
E: You don't have enough free space in /var/cache/apt/archives/.
So let's try to write 1 GB to /var/cache/apt/archives/:
container# dd if=/dev/urandom of=/var/cache/apt/archives/bigfile.img
bs=1M count=1024
sync
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 7.66032 s, 140 MB/s
container# sync
container# rm /var/cache/apt/archives/bigfile.img
This IMO is a filesystem bug?
Some more info:
# btrfs fi df /data
Data, single: total=580.01GiB, used=569.78GiB
System, single: total=4.00MiB, used=80.00KiB
Metadata, single: total=2.01GiB, used=1.53GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
# btrfs fi show /data
Label: 'data' uuid: 38fc3739-8695-49f1-bf14-946d0c0869c5
Total devices 1 FS bytes used 571.30GiB
devid 1 size 1000.00GiB used 582.02GiB path /dev/nvme1n1
Tomasz Chmielewski
https://lxadm.com
