Add an alert and a runbook for low disk space

I've picked 10% remaining free space as the threshold.

docs/src/SUMMARY.md

@@ -10,6 +10,7 @@
 
 # Alert Runbooks
 
+- [DiskSpaceLow](./runbooks/alerts/diskspacelow.md)
 - [ZPoolStatusDegraded](./runbooks/alerts/zpoolstatusdegraded.md)
 
 # "How to" Runbooks

docs/src/runbooks/alerts/diskspacelow.md

@@ -0,0 +1,35 @@
+DiskSpaceLow
+============
+
+This alert fires when a partition has under 10% free space remaining.
+
+The alert will say which partitions are affected, `df -h` also has the
+information:
+
+```
+$ df -h
+Filesystem                Size  Used Avail Use% Mounted on
+devtmpfs                  1.6G     0  1.6G   0% /dev
+tmpfs                      16G  112K   16G   1% /dev/shm
+tmpfs                     7.8G  9.8M  7.8G   1% /run
+tmpfs                      16G  1.1M   16G   1% /run/wrappers
+local/volatile/root       1.7T  1.8G  1.7T   1% /
+local/persistent/nix      1.7T  5.1G  1.7T   1% /nix
+local/persistent/persist  1.7T  2.0G  1.7T   1% /persist
+local/persistent/var-log  1.7T  540M  1.7T   1% /var/log
+efivarfs                  128K   40K   84K  33% /sys/firmware/efi/efivars
+local/persistent/home     1.7T   32G  1.7T   2% /home
+/dev/nvme0n1p2            487M   56M  431M  12% /boot
+data/nas                   33T   22T   11T  68% /mnt/nas
+tmpfs                     3.2G   12K  3.2G   1% /run/user/1000
+```
+
+Note all ZFS datasets in the same pool (`local/*` and `data/*` in the example
+above) share the underlying storage.
+
+Debugging steps:
+
+- See the `node_filesystem_avail_bytes` metric for how quickly disk space is
+  being consumed
+- Use `ncdu -x` to work out where the space is going
+- Buy more storage if need be

shared/default.nix

@@ -146,16 +146,6 @@ in
     services.zfs.autoSnapshot.enable = thereAreZfsFilesystems;
     services.zfs.autoSnapshot.monthly = 3;
 
-    services.prometheus.rules = mkIf thereAreZfsFilesystems [
-      ''
-        groups:
-        - name: zfs
-          rules:
-          - alert: ZPoolStatusDegraded
-            expr: node_zfs_zpool_state{state!="online"} > 0
-      ''
-    ];
-
     # Actually panic when ZFS "panics"
     # https://utcc.utoronto.ca/~cks/space/blog/linux/ZFSPanicsNotKernelPanics
     boot.extraModprobeConfig = mkIf thereAreZfsFilesystems ''
@@ -273,6 +263,20 @@ in
       };
     };
 
+    services.prometheus.rules = [
+      ''
+        groups:
+        - name: disk
+          rules:
+          - alert: DiskSpaceLow
+            expr: node_filesystem_avail_bytes{fstype!~"(ramfs|tmpfs)"} / node_filesystem_size_bytes < 0.1
+        - name: zfs
+          rules:
+          - alert: ZPoolStatusDegraded
+            expr: node_zfs_zpool_state{state!="online"} > 0
+      ''
+    ];
+
     # Host metrics
     services.prometheus.exporters.node.enable = promcfg.enable;