README.md

@@ -276,6 +276,8 @@ zpool status zpool
 
 # ZFS setup explained
 
+## Overview
+
 The ZFS pool and dataset setup that makes this tick, explained in plain English.
 
 1. Create zpool with options:
@@ -310,6 +312,41 @@ The ZFS pool and dataset setup that makes this tick, explained in plain English.
 
 The complete ZFS structure now exists and is mounted at `/mnt` ready for any `pacstrap`, [debootstrap](https://wiki.debian.org/Debootstrap), `dnf --installroot` or other bootstrapping action.
 
+## Adding another boot environment-independent dataset
+
+Assume that in addition to your `/home` data which lives on `zpool/data/home` you want another dataset that is exempt from Arch Linux snapshots.
+
+Consider an example `/opt/git` directory where a bunch of Git repos are checked out on which you work. You don't want them to be snapshotted - and rolled back - when something goes sideways: they are decoupled from everything else that goes on on your machine so you can easily and safely have a static `/opt/git` directory available in all boot environments.
+
+Move your current `/opt/git` data out of the way for a moment:
+```
+mv '/opt/git'{,'.bak'}
+```
+Create datasets
+```
+zfs create -o canmount=off zpool/data/opt
+zfs create zpool/data/opt/git
+```
+Remember that the `zpool/data` dataset already exists and has that it has both `mountpoint=/` and `canmount=off` set. It is not and cannot be mounted itself, it instead conveniently anchors datasets at `/`. Since the `canmount` dataset property cannot be inherited and defaults to `canmount=on` we have to manually specify `-o canmount=off`. Our new `zpool/data/opt` should not automatically mount into `/opt`.
+
+We then create the child dataset `zpool/data/opt/git`, it defaults to `canmount=on` thus immediately shows up at `/opt/git`.
+
+Move data back into place and clean up temp directory
+```
+rsync -av --remove-source-files '/opt/git'{'.bak',}
+find '/opt/git.bak' -type d -empty -delete
+```
+
+An example `zpool/data` dataset may now look like so:
+```
+# zfs list -r -oname,mountpoint,canmount zpool/data
+NAME                MOUNTPOINT  CANMOUNT
+zpool/data          /           off
+zpool/data/home     /home       on
+zpool/data/opt      /opt        off
+zpool/data/opt/git  /opt/git    on
+```
+
 # Development
 
 ## Conventional commits