quota-exporter

A prometheus exporter that reports numbers from Linux disk quota.

The exporter needs to be pointed at a list of mountpoints for which quotas should be reported. Both current usage and configured limits are reported in the metrics.

Run the exporter with the following command:

quota-exporter --mountpoint /home

Multiple mountpoints can be included by repeating the --mountpoint option. Alternatively, you can also run quota-exporter with the --all flag, which will cause it to export quotas for all mountpoints that support it.

By default the exporter will listen on port 10018 and export the metrics under the /metrics path. The port can be overriden using the --listen option.

The exporter needs to run as root (or with CAP_SYS_ADMIN). This is required by the kernel's quota interface.

Currently only user quotas are supported.

Testing

Configuring quotas requires modification to the host system to mount new disks and enable quotas, as well as adding new users. It's not clear how reliable this would be, even in the context of a Docker container.

Instead the tests are implemented as a QEMU VM, using the NixOS test framework. See the test.nix file for details.

The tests can be run with the following command:

nix run .#vm-test

Compatibility

Kernel version

The implementation uses the quotactl_fd system call, which was introduced in Linux 5.14. This is available in Ubuntu 22.04 and above, as well as Debian bookworm and above.

It would be possible to support older kernel version by using the existing quotactl system call, though the old interface is more tedious to use.

The old system call expects the path to the block device as an argument, which requires the application to figure out which block device is behind the given mount point. The new system call expects an O_PATH file descriptor of the mountpoint instead.

Filesystems

This has only been tested against ext4 filesystems. Other file systems may not work out of the box.

On-disk quota files

Linux supports two ways of storing quota data on an ext4 filesystem. They can either be stored as a regular file (often called aquota.user) or as a hidden inode. The latter can be enabled by passing -O quota to mkfs.ext4 or tune2fs.

quota-exporter should work with either schemes, but using the hidden inode is generally more reliable and is recommended.