docs(zfs): Display mounted state in ZFS example (#3)

README.md

@@ -341,12 +341,61 @@ 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
+# zfs list -r -oname,mountpoint,canmount,mounted zpool/data
+NAME                MOUNTPOINT       CANMOUNT  MOUNTED
+zpool/data          /                off       no
+zpool/data/home     /home            on        yes
+zpool/data/opt      /opt             off       no
+zpool/data/opt/git  /opt/git         on        yes
+```
+
+## Nested environment-independent datasets
+
+### Caution
+
+If you want a dedicated dataset for a directory that lives deeper in your file system tree than just `/opt/git`, for example like `/var/lib/docker` make sure to not recursively create this structure in a single `zfs create` command.
+
+In [Adding another boot environment-independent dataset](#adding-another-boot-environment-independent-dataset) above you can safely do:
+```
+zfs create -o canmount=off zpool/data/opt
+```
+Here `zpool/data` already exists, you're only creating one child dataset `opt` and you're setting `-o canmount=off` so that it never mounts into your `/opt` directory.
+
+Now consider the same setup for `/var/lib/docker`. If you follow the exact same approach:
+```
+zfs create -o canmount=off zpool/data/var/lib
+```
+Docker will correctly report:
+```
+cannot create 'zpool/data/var/lib': parent does not exist
+```
+You might want to just create the parent then with `-p` argument:
+```
+zfs create -p -o canmount=off zpool/data/var/lib
+           ~~
+```
+Note, however, that `-o canmount=off` only applies to `lib` dataset and that `zpool/data/var` has just been auto-mounted into `/var`:
+```
+# zfs list -r -oname,mountpoint,canmount,mounted zpool/data
+NAME                MOUNTPOINT  CANMOUNT  MOUNTED
+zpool/data          /           off       no
+zpool/data/home     /home       on        yes
+zpool/data/opt      /opt        off       no
+zpool/data/opt/git  /opt/git    on        yes
+zpool/data/var      /var        on        yes  <---
+zpool/data/var/lib  /var/lib    off       no
+```
+
+### Advice
+
+Instead create nested parents in multiple steps where you set each one to `-o canmount=off`:
+```
+zfs create -o canmount=off zpool/data/var
+zfs create -o canmount=off zpool/data/var/lib
+```
+Lastly create the dataset you want mounted:
+```
+zfs create zpool/data/var/lib/docker
 ```
 
 # Development