Like smartd, Debian installs btrfsmaintenance in a disabled state.
One day, after consensus on what the most useful and safe default
configuration has been reached, maintenance operations will be
automatically enabled.

Configuration is found at /etc/default/btrfsmaintenance

Btrfsmaintenance reads this configuration and schedules maintenance
every boot by running btrfsmaintenance-refresh.service when a
systemd-enabled system is booted in multi-user mode.  To reload
configuration manually (recommended) run:

  systemctl restart btrfsmaintenance-refresh.service

To refresh scheduling on every boot run:

  systemctl enable btrfsmaintenance-refresh.service

Enabling that service will also enable btrfsmaintenance-refresh.watch
if is enabled by a normal user who then authenticates for
org.freedesktop.systemd1.manage-unit-files (root).  Refresh.path
applies scheduling changes whenever /etc/default/btrfsmaintenance is
modified.  To disable this automatic behaviour run:

  systemctl disable btrfsmaintenance-refresh.watch

Finally, enable systemd timers:

  systemctl enable btrfs-balance.timer  # recommended
  systemctl enable btrfs-defrag.timer   # recommended
  systemctl enable btrfs-scrub.timer    # recommended
  systemctl enable btrfs-trim.timer     # not recommended


For the time being, users who forgo systemd can enable roughly
equivalent behaviour by running the following script directly:

  /usr/share/btrfsmaintenance/btrfsmaintenance-refresh-cron.sh

To enable this on every boot, add it to /etc/rc.local.  To refresh
configuration every hour, run:

  cd /etc/cron.hourly
  ln -s ../../usr/share/btrfsmaintenance/btrfsmaintenance-refresh-cron.sh ./

Btrfsmaintenance must not be configured to use both the systemd and
cron methods (see upstream README).  One day, when systemd units for
btrfsmaintenance are automatically activated, those who choose not to
use systemd will still need to manually activate the refresh-cron
script, because upstream maintains that providing automatic scheduling
with cron conflicts with automatically enabled scheduling with
systemd.

 -- Nicholas D Steeves <nsteeves@gmail.com>, Sun,  4 Feb 2018 15:59:20 -0500