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Setting max_allowed_packet
When a MySQL client or the mysqld server gets a packet bigger than max_allowed_packet bytes, it issues a Packet too large error and closes the connection.
In MySQL 3.23 the biggest possible packet is 16M (due to limits in the client/server protocol). In MySQL 4.0.1 and up, this is only limited by the amount on memory you have on your server (up to a theoretical maximum of 2G).
A communication packet is a single SQL statement sent to the MySQL server or a single row that is sent to the client.
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 1G in MySQL 4.0.
it does not makes sense to set it higher than 1G on 32bit boxes as 3*max_allowed_packet memory might be allocated for row processing. So current real limit for the packet size is 1G, and practical limit is even lower.
On 64bit systems there shall be no problems up to higher values.