max_allowed_packet The maximum size of one packet.
The message buffer is initialised to net_buffer_length bytes,
but can grow up to max_allowed_packet bytes when needed.
This value by default is small, to catch big (possibly wrong) packets.
You must increase this value if you are using big BLOB columns.
It should be as big as the biggest BLOB you want to use.
The protocol limits for max_allowed_packet is 16M in MySQL 3.23
and 2G in MySQL 4.0.
 http://www.mysql.com/doc/S/H/SHOW_VARIABLES.html  버전에 따라서 다르네요. 4.0은 2G 라는군요.