btrbk software package provides backup tool for btrfs subvolumes, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install btrbk
btrbk is installed in your system.
Make ensure the btrbk package were installed using the commands given below,
$ sudo dpkg-query -l | grep btrbk *
You will get with btrbk package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Utilities |
| Package | btrbk |
| Version | 0.23.3-1 |
| Description | backup tool for btrfs subvolumes |
| Sub Section | Utils |