private void extendMapping() throws java.io.IOException {
        if (ctlFile == null) return;
        long pos = 0;
        long len = ctlFile.length();
        if (mapBuf == null || mapBuf.capacity() != len) {
            mapBuf = ctlFile.getChannel().map(FileChannel.MapMode.READ_ONLY, pos, len);
        }
        levelControllerRepo = new MappedLevelControllerRepo(mapBuf, controlFileHeaderLength, numLevels, logControlFilename);
    }
level controller for default levels (first-time logging
or logging without a logcontrol file)
vespalog/src/main/java/com/yahoo/log/VespaLevelControllerRepo.java