diff --git a/modules/btrbk/README.md b/modules/btrbk/README.md
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+# btrbk
+
+* https://digint.ch/btrbk/doc/readme.html
+* https://digint.ch/btrbk/doc/btrbk.1.html
+* https://digint.ch/btrbk/doc/btrbk.conf.5.html
+
+> btrbk - backup tool for btrfs subvolumes
+
+- Take snapshots (on same disk)
+- Create multiple backups (on different disks) over SSH
+
+## Snapshots and Backups
+
+subvolume: <subvolume-name>
+
+snapshot: <snapshot-name>.<timestamp>[<N>]
+
+timestamp: Can be configured using the `timestamp_format` option.
+
+N: If multiple snapshots are created at the same time, the increasing counter N differentiates them.
+
+## Actions
+
+`run [filter...]`: In multiple steps, perform snapshots and backups.
+* Read Data: Perform sanity checks
+* Create Snapshots
+* Create Backups
+* Delete Backups
+* Delete Snapshots
+
+`clean [filter...]`: Delete incomplete backups (network error, kill signal while send/receive was ongoing).
+
+## Configuration
+
+### General
+
+`volume <volume-directory|<ulr>`: Usually the mount point of a btrfs filesystem mounted with `subvolid=5`.
+
+`subvolume <subvolume-name|<url>`: Subvolume to be backed up, relative to `<volume-directory>`.
+
+`snapshot_dir <directory>`: Directory in which the btrfs snapshots are created, relative to `<volume-directory>`. Hast to be created **manually**!
+
+`target <target-directory>|<url>`: Target directory where the backup subvolumes are to be created. Multiple target sections are allowed.
+
+`url`: Accepted formats are:
+* `ssh://<hostname>[:<port>]/<directory>`
+* `<hostname>:<directory>`
+
+`hostname`: Either a host name, IPv4 or IPv6 address
+
+`timestamp_format short|long|long-iso`: This becomes relevant for setups with multiple btrbk instances, e.g. many snapshot-only instances (spread around the world), and a fetch-only instance on the backup server. Make sure to run btrbk with the same time zone on every host, e.g. by setting the TZ environment variable (see tzset(3)).
+
+### Retention
+
+`snapshot_preserve <retention_policy>`
+
+`snapshot_preserve_min all|latest|<number>{h,d,w,m,y}`: Preserve all snapshots for a minimum amount of hours (h), days (d), weeks (w), months (m) or years (y).
+
+`target_preserve <retention_policy>`
+
+`target_preserve_min all|latest|no|<number>{h,d,w,m,y}`: Preserve all snapshots for a minimum amount of hours (h), days (d), weeks (w), months (m) or years (y). If set to "no", only the backups following the `target_preserve` policy are created.
+
+`<retention_policy>`: `[<hourly>h] [<daily>d] [<weekly>w] [<monthly>m] [<yearly>y]`. Defines how many backups of each period should be preserved.
+
+### SSH and Data Stream Options
+
+`ssh_compression yes|no`
+
+`stream_compress <compress_command>`: Compress the btrfs send stream before transferring it from/to remote locations.
+
+`<compress_command>`: gzip, pigz, bzip2, pbzip2, bzip3, xz, lzo, lz4, zstd.
+
+`stream_buffer <size>|no`: This can give a speed improvement (measured up to 20%) on both local or remote operations, but also increases system load. Make sure that the `mbuffer` command is available. Leave this option disabled if your main concern is a stable backup process: while recent versions of mbuffer have proven reliable, it is often desirable to keep things simple rather than adding an additional, multi-threaded process to the command pipe.
+
+`rate_limit <rate>|no`: Limit the read rate of the btrfs send stream to `<rate>` bytes per second. Implies the `stream_buffer` option.
+
+### System Options
+
+`transaction_syslog <facility>|no`: Log all transactions to syslog.
+
+`lockfile <file>|no`
+
+## Example: Local Snapshots and Backup
+
+```
+timestamp_format      long
+
+snapshot_preserve_min 2d
+snapshot_preserve     24h 7d 4w 6m
+
+target_preserve_min   no
+target_preserve       7d 4w 6m
+
+volume /mnt/12tb1-top-level
+  snapshot_dir @snap
+  target /mnt/12tb2-top-level/@snap
+  subvolume    @home
+  subvolume    @
+```
+
+Mount top-level subvolume:
+
+```shell
+mount -o subvolid=5 /dev/mapper/some-disk /mnt/12tb1-top-level
+```
+
+Manually create snapshot directory:
+
+```shell
+btrfs subvolume create /mnt/12tb1-top-level/@snap
+btrfs subvolume create /mnt/12tb2-top-level/@snap
+```
+
+Snapshots only:
+
+```shell
+btrbk snapshot
+```
+
+Backup only:
+
+```shell
+btrbk resume
+```
+
+Create an additional snapshot of @home:
+
+```shell
+btrbk snapshot @home
+```
+
+## Testing on Local Host
+
+```shell
+sudo mkdir -p /mnt/top-level
+ls /mnt/top-level/@ || sudo mount -o subvolid=5 /dev/mapper/luks-ea7099e3-320d-4eb3-a4c3-9910a9af817b /mnt/top-level
+ls /mnt/top-level/@snap || sudo btrfs subvolume create /mnt/top-level/@snap
+ls /mnt/top-level/@foo || {
+  sudo btrfs subvolume create /mnt/top-level/@foo
+  sudo touch /mnt/top-level/@foo/bar
+}
+```
+
+```shell
+ls ~/btrbk-usb.img || {
+  fallocate -l 1G ~/btrbk-usb.img
+  sudo mkfs.btrfs ~/btrbk-usb.img
+  # Mount and create subvolume @
+}
+
+sudo mkdir -p /mnt/usb-top-level
+ls /mnt/usb-top-level/@ || sudo mount -o subvolid=5 ~/btrbk-usb.img /mnt/usb-top-level
+ls /mnt/usb-top-level/@snap || sudo btrfs subvolume create /mnt/usb-top-level/@snap
+}
+```
+
+```shell
+cat > ~/btrbk.cfg <<'EOF'
+timestamp_format      long
+
+snapshot_preserve_min 2d
+snapshot_preserve     24h 7d 4w 6m
+
+target_preserve_min  no
+target_preserve      7d 4w 6m
+
+volume /mnt/top-level
+  snapshot_dir @snap
+  target /mnt/usb-top-level/@snap
+  subvolume    @foo
+EOF
+```
+
+```shell
+# Check configuration.
+sudo btrbk -c ~/btrbk.cfg config print
+sudo btrbk -c ~/btrbk.cfg config list
+
+
+# Dry run.
+sudo btrbk -c ~/btrbk.cfg run -n
+# Create first snapshot.
+sudo btrbk -c ~/btrbk.cfg run
+# Print schedule.
+sudo btrbk -c ~/btrbk.cfg run -n -S
+
+sleep 5m
+sudo btrbk -c ~/btrbk.cfg run
+
+# List snapshots
+sudo btrbk -c ~/btrbk.cfg list
+```
+
+### NixOS Config
+
+```nix
+{ config, pkgs, ... }:
+{
+  services.btrbk = {
+    extraPackages = [ pkgs.lz4 ];
+    # Lowest scheduling priority.
+    niceness = 19;
+    # Set of btrbk instances. The instance named btrbk is the default one.
+    instances = {
+      # This is the configuration equivalent to
+      # README.md section "Testing on Local Host".
+      "testing-on-local-host" = {
+        # The timer is disabled. The service has to be invoked manually.
+        onCalendar = null;
+        #onCalendar = "hourly";
+        #onCalendar = "*:0/15"; # Every 15min
+        # Configuration options for btrbk. Nested attrsets translate to subsections.
+        settings = {
+          timestamp_format = "long";
+          stream_compress = "lz4";
+
+          snapshot_preserve_min = "2d";
+          snapshot_preserve = "24h 7d 4w 6m";
+
+          target_preserve_min = "no";
+          target_preserve = "7d 4w 6m";
+
+          volume."/mnt/top-level" = {
+            snapshot_dir = "@snap";
+            target = "/mnt/usb-top-level/@snap";
+            subvolume = "@foo";
+          };
+        };
+      };
+    };
+  };
+}
+```
+
+## pi3bplus Host Config
+
+A local `btrbk` instance manages snapshots and local backups.
+
+Another remote backup server periodically pulls snapshots to create remote backups. Here is its config:
+
+```shell
+cat > ~/btrbk.cfg <<'EOF'
+timestamp_format    long
+ssh_identity        /mnt/backup/rootnas_ed25519
+
+target_preserve_min no
+# Daily backups -> @daily cron entry
+target_preserve          7d 4w 6m
+# Hourly backups -> @hourly cron entry
+#target_preserve     24h 7d 4w 6m
+
+# Don't create or delete snapshots on remote server "rootnas".
+snapshot_preserve_min  all
+snapshot_create        no
+
+volume ssh://rootnas/mnt/data/data
+  snapshot_dir /mnt/data/snap
+  target       /mnt/backup/snap
+  subvolume    arch.p1st.de
+  subvolume    blogger.privacy1st.de
+  subvolume    changedetection.p1st.de
+  subvolume    cloud.media-kollektiv.eu
+  subvolume    cloud.privacy1st.de
+  subvolume    git.privacy1st.de
+  subvolume    money.p1st.de
+  subvolume    music.privacy1st.de
+  subvolume    proxy
+  subvolume    recipe.privacy1st.de
+  subvolume    traggo.privacy1st.de
+EOF
+```
diff --git a/modules/btrbk/default.nix b/modules/btrbk/default.nix
index 8afb8aa..839d271 100644
--- a/modules/btrbk/default.nix
+++ b/modules/btrbk/default.nix
@@ -45,32 +45,6 @@
     # Set of btrbk instances. The instance named btrbk is the default one.
     instances = {
 
-#      # This is the configuration equivalent to
-#      # README.md section "Testing on Local Host".
-#      "testing-on-local-host" = {
-#        # The timer is disabled. The service has to be invoked manually.
-#        onCalendar = null;
-#        #onCalendar = "hourly";
-#        #onCalendar = "*:0/15"; # Every 15min
-#        # Configuration options for btrbk. Nested attrsets translate to subsections.
-#        settings = {
-#          timestamp_format = "long";
-#          stream_compress = "lz4";
-#
-#          snapshot_preserve_min = "2d";
-#          snapshot_preserve = "24h 7d 4w 6m";
-#
-#          target_preserve_min = "no";
-#          target_preserve = "7d 4w 6m";
-#
-#          volume."/mnt/top-level" = {
-#            snapshot_dir = "@snap";
-#            target = "/mnt/usb-top-level/@snap";
-#            subvolume = "@foo";
-#          };
-#        };
-#      };
-
       "local-snapshots" = {
         onCalendar = "hourly";
         #onCalendar = "*:0/15"; # Every 15min