Starting from BackupPC 4, if you back up client systems using the rsync method
you will need the backuppc-rsync package installed in order to take
full advantage of the rsync protocol including BackupPC-specific changes.
It is co-installable with the normal rsync Debian package.

Additionally you'll need to replace all occurrences of the removed
$Conf{RsyncClientCmd} setting with $Conf{RsyncSshArgs} and
possibly $Conf{RsyncClientPath} with accordingly updated values.

You also need to update /etc/backuppc/config.pl and add the line:

  $Conf{RsyncBackupPCPath} = "/usr/libexec/backuppc-rsync/rsync_bpc";

Example: If you used $Conf{RsyncClientCmd} to configure the user with
which BackupPC should login on the remote host and a remote nice level,
replace

$Conf{RsyncClientCmd} = '$sshPath -l root $host nice -n 10 $rsyncPath $argList+';

with those two lines like these in the client configuration file:

$Conf{RsyncSshArgs} = ['-e', '$sshPath -l root'];
$Conf{RsyncClientPath} = 'nice -n 10 /usr/bin/rsync';

See also "V3 migration" in the backuppc(8) man page or in
/usr/share/doc/backuppc/BackupPC.html as well as
https://github.com/backuppc/backuppc/issues/132.
