One thing I’ve not figured out yet is this: How does enabling btrfs quotas impact the system? – Unix & Linux Stack Exchange
My gut feeling says it’s not much. But if someone knows: please chime in.
Anyway: btrfs quota support helps you to see how big snapshots (and other subvolumes) are.
Enabling and disabling is easy (the slash is the path from where to enable quota):
btrfs quota enable /
btrfs quota disable /
The bad thing is that there is no command that gives just the quota status. You have to revert to btrfs qgroup show /
This will show you quota groups when quota are enabled:
qgroupid rfer excl -------- ---- ---- 0/5 16.00KiB 16.00KiB
or errors when quota are disabled either of these two errors:
# btrfs qgroup show / ERROR: can't perform the search - No such file or directory ERROR: can't list qgroups: No such file or directory # btrfs qgroup show / ERROR: can't list qgroups: quotas not enabled
You might also want to check the syslogs as there was a [PATCH 1/2] btrfs: syslog when quota is enabled.
The btrfs qgroup output is the base for agronick/btrfs-size: A script that will print out a list of BTRFS subvolumes along with their size in megabytes and their name.
It actually shows it in human readable form (kilo/mega/giga/… bytes) and is documented at Get the size of your BTRFS Snapshots | PoisonPacket. It gives me a nice overview that corresponds to the snapper list output (see below).
The script has two drawbacks:
- when quota is disabled, you get this error:
./btrfs-size/btrfs-size.sh: line 55: can't + 0: syntax error: invalid arithmetic operator (error token is "'t + 0")
- it can take a while for
btrfs quota enable /to actually deliver the quota for all the qgroupid values snapper uses. Until that time, the columns ID/Total/Exclusive Data are empty. - it won’t warn you if the quota data is out of data; to check that, run the below script
# btrfs qgroup show / > /dev/null
WARNING: Qgroup data inconsistent, rescan recommended
In that case, run btrfs quota rescan / and wait for it to complete by running this command until the warning is gone:
# btrfs qgroup show / > /dev/null
WARNING: Rescan is running, qgroup data may be incorrect
If the warning reverts to WARNING: Qgroup data inconsistent, rescan recommended then you might see the effects from this bug and should not worry about it: Bug 933428 – btrfs qgroup – allways shows WARNING: Qgroup data inconsistent.
For more btrfs quota background information, see: UseCases – btrfs Wiki.
–jeroen
The Wiert Corner - irregular stream of stuff 
