[syslog-ng] log_disk_fifo_size() option available in syslog-ng 3.1.1 ?

Hello, I tried to use the disk-based buffering as described in the document "The syslog-ng Open Source Edition 3.1 Administrator.pdf" but I have a syntax error when I start the process. I used this configuration for the destination part of my configuration file : destination d_tcp { tcp("10.1.2.3" port(1999) log_disk_fifo_size(4194304)); }; When I remove "log_disk_fifo_size" of my configuration, everything is OK and syslog-ng doesn't complain. Did I miss something ? Regards, Yann