NixOS Root on ZFS: use Nix-config for multi-disk

NixOS has enjoyed popularity among ZFS users thanks to
its declarative configuration and native ZFS support.
However, the installation guide used hardcoded disk
names in configuration files, which is unnecessary and
is the source of difficulties in multidisk setups.

The guide is now rewritten to leverage expressions in
the Nix language to manage multidisk setups.

Also adds instruction on replacing failed disk.

Closes #385.

Signed-off-by: Maurice Zhou <ja@apvc.uk>

docs/Getting Started/NixOS/Root on ZFS/2-system-installation.rst

@@ -16,7 +16,7 @@ System Installation
 
      sgdisk -n2:0:+4G -t2:BE00 $i
 
-     test -z $INST_PARTSIZE_SWAP || sgdisk -n4:0:+${INST_PARTSIZE_SWAP}G -t4:8200 $i
+     sgdisk -n4:0:+${INST_PARTSIZE_SWAP}G -t4:8200 $i
 
      if test -z $INST_PARTSIZE_RPOOL; then
          sgdisk -n3:0:0   -t3:BF00 $i
@@ -25,6 +25,12 @@ System Installation
      fi
 
      sgdisk -a1 -n5:24K:+1000K -t5:EF02 $i
+
+     sync && udevadm settle && sleep 3 
+
+     cryptsetup open --type plain --key-file /dev/random $i-part4 ${i##*/}-part4
+     mkswap /dev/mapper/${i##*/}-part4
+     swapon /dev/mapper/${i##*/}-part4 
      done
 
 #. Create boot pool::
@@ -84,9 +90,7 @@ System Installation
 
    If not using a multi-disk setup, remove ``mirror``.
 
-#. This section implements dataset layout as described in `overview <1-preparation.html>`__.
-
-   Create root system container:
+#. Create root system container:
 
    - Unencrypted::
 
@@ -108,18 +112,25 @@ System Installation
        -o keyformat=passphrase \
        rpool/nixos
 
-   Create system datasets::
+   You can automate this step (insecure) with: ``echo POOLPASS | zfs create ...``.
 
-      zfs create -o canmount=on -o mountpoint=/     rpool/nixos/root
-      zfs create -o canmount=on -o mountpoint=/home rpool/nixos/home
-      zfs create -o canmount=off -o mountpoint=/var  rpool/nixos/var
-      zfs create -o canmount=on  rpool/nixos/var/lib
-      zfs create -o canmount=on  rpool/nixos/var/log
+   Create system datasets, let NixOS declaratively
+   manage mountpoints with ``mountpoint=legacy``::
 
-   Create boot dataset::
-
-     zfs create -o canmount=off -o mountpoint=none bpool/nixos
-     zfs create -o canmount=on -o mountpoint=/boot bpool/nixos/root
+      zfs create -o mountpoint=legacy     rpool/nixos/root
+      mount -t zfs rpool/nixos/root /mnt/
+      zfs create -o mountpoint=legacy rpool/nixos/home
+      mkdir /mnt/home
+      mount -t zfs  rpool/nixos/home /mnt/home
+      zfs create -o mountpoint=legacy  rpool/nixos/var
+      zfs create -o mountpoint=legacy rpool/nixos/var/lib
+      zfs create -o mountpoint=legacy rpool/nixos/var/log
+      zfs create -o mountpoint=none bpool/nixos
+      zfs create -o mountpoint=legacy bpool/nixos/root
+      mkdir /mnt/boot
+      mount -t zfs bpool/nixos/root /mnt/boot
+      zfs create -o mountpoint=legacy rpool/nixos/empty
+      zfs snapshot rpool/nixos/empty@start
 
 #. Format and mount ESP::