As specified at
https://www.freedesktop.org/software/systemd/man/systemd.timer.html#Persistent=
the service will be activated immediately on machine boot if it would
have been triggered when the machine was turned off. This makes it
possible for the service to be activated before remote hosts are
available. Adding the `After` directive should make sure that it is
only triggered after remote hosts are reachable.
When used without --inplace, rsync creates a new copy of the file and
moves it into place when it is complete, having the effect that btrfs
creates a new extent for the WHOLE file. With --inplace however, rsync
writes the updated data directly to the destination file, having the
effect that btrfs creates a new extent only for the differing part of
the file.
Jonas Kalderstam