Add blockdevice example

src/SUMMARY.md

@@ -28,6 +28,7 @@
   - [Comparing Snapshots](./commands/backup/comparing_snapshots.md)
   - [Special Items and Metadata](./commands/backup/special_items_metadata.md)
   - [Reading data from StdIn](./commands/backup/reading_stdin.md)
+  - [Backup Block Devices](./commands/backup/blockdev.md)
   - [Tags](./commands/backup/tags.md)
   - [Scheduling](./commands/backup/scheduling.md)
   - [Space requirements](./commands/backup/space_requirements.md)

src/commands/backup/blockdev.md

@@ -0,0 +1,31 @@
+# Backup block devices
+
+Special files like block devices or pipes are by default stored by only the
+metadata of the special file like a device number. This allows to restore
+something like `/dev` on Unix platforms and to get the original state.
+
+Sometimes, however, you want to backup the contents of block device, e.g. a
+snapshot of a disc. To do so, rustic supports the `set-devid` options. You can
+define it in the config file:
+
+```toml
+[[backup.snapshots]]
+sources = ["/dev/my-blockdev"]
+set-blockdev = "file"
+```
+
+or directly on the command line
+
+```console
+rustic backup --set-blockdev=file /dev/my-blockdev
+```
+
+Note that for blockdevices, a fixed sized chunking with chunksize being a
+multiple of the block size is recommended as this speeds-up the backup from
+large single files and also good chunking points are typically on block
+boundaries for block devices. Fixed size chunking can be set e.g. during
+repository initalization, for example:
+
+```console
+rustic init --set-chunker=fixed_size --set-chunk-size=1MiB
+```