I did not succeed to specify a relative path for the logfile parameter. Redis creates the log file but the file remains empty. Redis complains, if the file could not be created (e.g. non existing parent directory or wrong permissions).
If I specify an absolute path name logging works.
But: specifying the PID file using a relative path works.
And please clarify that the PID file needs to be specified relatively to the redis-server binary. But the log file needs to be specified relative to the directory declared by the directory parameter.
... and that the directory parameter is relative to CWD