node_filesystem_{size,avail}_bytes report wrong values for ZFS filesystems

Host operating system: output of uname -a

Linux backup-target.atelieraphelion.com 5.0.0-29-generic #31~18.04.1-Ubuntu SMP Thu Sep 12 18:29:21 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

node_exporter version: output of node_exporter --version

/nix/store/svm3ypaq5dyznfxr7lhqvk6ymyy0cs9n-node_exporter-0.17.0-bin/bin/node_exporter --version
node_exporter, version (branch: , revision: )
build user:
build date:
go version: go1.12.7

node_exporter command line flags

/nix/store/svm3ypaq5dyznfxr7lhqvk6ymyy0cs9n-node_exporter-0.17.0-bin/bin/node_exporter --web.listen-address 0.0.0.0:9100

Are you running node_exporter in Docker?

No.

What did you do that produced an error?

Created a disk-space alert using node_filesystem_avail_bytes{fstype=~"ext4|zfs|xfs"} / node_filesystem_size_bytes < 0.1

What did you expect to see?

The alert should fire if available space is below 10%, as _avail_bytes should be <10% of size_bytes.

What did you see instead?

The alert never fires for ZFS filesystems, because _avail_bytes and _size_bytes are always equal.

Start by locating the node_exporter filesystem collector and reproducing the reported metrics for a ZFS filesystem using node_filesystem_avail_bytes and node_filesystem_size_bytes. Done means the exporter reports available and total ZFS space separately so the stated alert can detect available space below 10%.