Some notes-to-self about adding new disks to the storage pool

nix/system/svalbard/README.md

@@ -1,8 +1,10 @@
+## ZFS
 To set up ZFS on the spinning discs, followed steps derived from these sources (but without encryption):
 * https://ipetkov.dev/blog/installing-nixos-and-zfs-on-my-desktop/
 * https://cheat.readthedocs.io/en/latest/nixos/zfs_install.html
 
-```
+```sh
+# zpool status
 # DISK=<disk from /dev/disk/by-id>
 # POOL=<a name for the pool>
 # zpool create "${POOL}" $DISK
@@ -12,6 +14,30 @@ To set up ZFS on the spinning discs, followed steps derived from these sources (
 then added the ZFS filesystem to `hardware-configuration.nix` (use the zfs
 created label as the device) and rebooted.
 
+### Adding a new disk to the pool
+
+Based on https://unix.stackexchange.com/questions/530968/adding-disks-to-zfs-pool, it should be straightforward
+as long as what you're trying to do is simply extend a pool's storage by adding a new vdev:
+
+```sh
+# fdisk -l
+# ls -l /dev/disk/by-id
+# DISK=<the new disk, based on which one is pointed to the right /dev/sdX>
+# fdisk $DISK
+# zpool status
+# zpool add <pool> $DISK
+```
+
+Since we've chosen single-disks for the storage pool, it's going to be annoying
+if we ever decide to go for any redundancy like raidz. We'll have to buy quite
+a few disks and copy over all the data :sob:. That is a good prompt to cull
+un-valued data!
+
+Someday, it might be possible to grow a raidz vdev by adding disks, instead of
+by replacing disks: https://github.com/openzfs/zfs/pull/12225
+
+## NFS
+
 Don't forget to `chown` the mounted system so that non-root can read/write there.