public static void createDirectory(File d) {
        if (d.exists()) {
            if (!d.isDirectory()) {
                throw new IllegalArgumentException(d.getAbsolutePath() + " exists, but isn't a directory.");
            }
        } else {
            if (!d.mkdirs()) {
                throw new IllegalArgumentException("Couldn't create " + d.getAbsolutePath());
            }
        }
    }
Config server db is the maintainer of the serverdb directory containing def files and the file system sessions.
See also {@link com.yahoo.vespa.config.server.deploy.TenantFileSystemDirs} which maintains directories per tenant.

@author Ulf Lilleengen
configserver/src/main/java/com/yahoo/vespa/config/server/ConfigServerDB.java