zrepl/docs/tutorial.rst

.. include:: global.rst.inc

.. _tutorial:

Tutorial
========


This tutorial shows how zrepl can be used to implement a ZFS-based pull backup.
We assume the following scenario:

* Production server ``app-srv`` with filesystems to back up:

  * ``zroot/var/db``
  * ``zroot/usr/home`` and all its child filesystems
  * **except** ``zroot/usr/home/paranoid`` belonging to a user doing backups themselves

* Backup server ``backup-srv`` with

  * Filesystem ``storage/zrepl/pull/app-srv`` + children dedicated to backups of ``app-srv``

Our backup solution should fulfill the following requirements:

* Periodically snapshot the filesystems on ``app-srv`` *every 10 minutes*
* Incrementally replicate these snapshots to ``storage/zrepl/pull/app-srv/*`` on ``backup-srv``
* Keep only very few snapshots on ``app-srv`` to save disk space
* Keep a fading history (24 hourly, 30 daily, 6 monthly) of snapshots on ``backup-srv``

Analysis
--------

We can model this situation as two jobs:

* A **source job** on ``app-srv``

  * Creates the snapshots
  * Keeps a short history of snapshots to enable incremental replication to ``backup-srv``
  * Accepts connections from ``backup-srv``

* A **pull job** on ``backup-srv``

  * Connects to the ``zrepl daemon`` process on ``app-srv``
  * Pulls the snapshots to ``storage/zrepl/pull/app-srv/*``
  * Fades out snapshots in ``storage/zrepl/pull/app-srv/*`` as they age


Why doesn't the **pull job** create the snapshots before pulling?

As is the case with all distributed systems, the link between ``app-srv`` and ``backup-srv`` might be down for an hour or two.
We do not want to sacrifice our required backup resolution of 10 minute intervals for a temporary connection outage.

When the link comes up again, ``backup-srv`` will happily catch up the 12 snapshots taken by ``app-srv`` in the meantime, without
a gap in our backup history.

Install zrepl
-------------

Follow the :ref:`OS-specific installation instructions <installation>` and come back here.

Configure ``backup-srv``
------------------------

We define a **pull job** named ``pull_app-srv`` in the |mainconfig|: ::

    jobs:
    - name: pull_app-srv
      type: pull
      connect:
          type: ssh+stdinserver
          host: app-srv.example.com
          user: root
          port: 22
          identity_file: /etc/zrepl/ssh/identity
      interval: 10m
      mapping: {
          "<":"storage/zrepl/pull/app-srv"
      }
      initial_repl_policy: most_recent
      snapshot_prefix: zrepl_pull_backup_
      prune:
          policy: grid
          grid: 1x1h(keep=all) | 24x1h | 35x1d | 6x30d

The ``connect`` section instructs the zrepl daemon to use the ``stdinserver`` transport:
``backup-srv`` will connect to the specified SSH server and expect ``zrepl stdinserver CLIENT_IDENTITY`` instead of the shell on the other side.

It uses the private key specified at ``connect.identity_file`` which we still need to create: ::

    cd /etc/zrepl
    mkdir -p ssh
    chmod 0700 ssh
    ssh-keygen -t ed25519 -N '' -f /etc/zrepl/ssh/identity

Note that most use cases do not benefit from separate keypairs per remote endpoint.
Thus, it is sufficient to create one keypair and use it for all ``connect`` directives on one host.

Learn more about :ref:`transport-ssh+stdinserver` transport and the :ref:`pull job <job-pull>` format.

.. _tutorial-configure-app-srv:

Configure ``app-srv``
---------------------

We define a corresponding **source job** named ``pull_backup`` in the |mainconfig|: ::

    jobs:
    - name: pull_backup
      type: source
      serve:
          type: stdinserver
          client_identity: backup-srv.example.com
      filesystems: {
          "zroot/var/db": "ok",
          "zroot/usr/home<": "ok",
          "zroot/usr/home/paranoid": "!",
      }
      snapshot_prefix: zrepl_pull_backup_
      interval: 10m
      prune:
          policy: grid
          grid: 1x1d(keep=all)


The ``serve`` section corresponds to the ``connect`` section in the configuration of ``backup-srv``.

We now want to authenticate ``backup-srv`` before allowing it to pull data.
This is done by limiting SSH connections from ``backup-srv`` to execute the ``stdinserver`` subcommand.

Open ``/root/.ssh/authorized_keys`` and add either of the the following lines.::

    # for OpenSSH >= 7.2
    command="zrepl stdinserver backup-srv.example.com",restrict CLIENT_SSH_KEY
    # for older OpenSSH versions
    command="zrepl stdinserver backup-srv.example.com",no-port-forwarding,no-X11-forwarding,no-pty,no-agent-forwarding,no-user-rc  CLIENT_SSH_KEY

.. ATTENTION::

    Replace CLIENT_SSH_KEY with the contents of ``/etc/zrepl/ssh/identity.pub`` from ``app-srv``.
    Mind the trailing ``.pub`` in the filename.
    The entries **must** be on a single line, including the replaced CLIENT_SSH_KEY.


.. HINT::

    You may need to adjust the ``PermitRootLogin`` option in ``/etc/ssh/sshd_config`` to ``forced-commands-only`` or higher for this to work.
    Refer to sshd_config(5) for details.

The argument ``backup-srv.example.com`` is the client identity of ``backup-srv`` as defined in ``jobs.serve.client_identity``.

Again, both :ref:`transport-ssh+stdinserver` transport and the :ref:`job-source` format are documented.

Apply Configuration Changes
---------------------------

We need to restart the zrepl daemon on **both** ``app-srv`` and ``backup-srv``.
This is :ref:`OS-specific <install-restarting>`.

Watch it Work
-------------

A common setup is to ``watch`` the log output and ``zfs list`` of snapshots on both machines.

If you like tmux, here is a handy script that works on FreeBSD: ::

    pkg install gnu-watch tmux
    tmux new-window
    tmux split-window "tail -f /var/log/zrepl.log"
    tmux split-window "gnu-watch 'zfs list -t snapshot -o name,creation -s creation | grep zrepl_pull_backup_'"
    tmux select-layout tiled

The Linux equivalent might look like this: ::

    # make sure tmux is installed & let's assume you use systemd + journald
    tmux new-window
    tmux split-window "journalctl -f -u zrepl.service"
    tmux split-window "watch 'zfs list -t snapshot -o name,creation -s creation | grep zrepl_pull_backup_'"
    tmux select-layout tiled

Summary
-------

Congratulations, you have a working pull backup. Where to go next?

* Read more about :ref:`configuration format, options & job types <configuration_toc>`
* Learn about :ref:`implementation details <implementation_toc>` of zrepl.