A communication packet is a single SQL statement sent to the
MySQL server or a single row that is sent to the client.
The largest possible packet that can be transmitted to or from a
MySQL 5.1 server or client is 1GB.
When a MySQL client or the mysqld server
receives a packet bigger than
max_allowed_packet bytes, it issues a
Packet too large error and closes the
connection. With some clients, you may also get a Lost
connection to MySQL server during query error if the
communication packet is too large.
Both the client and the server have their own
max_allowed_packet variable, so if you want
to handle big packets, you must increase this variable both in
the client and in the server.
If you are using the mysql client program,
its default max_allowed_packet variable is
16MB. To set a larger value, start mysql like
this:
mysql> mysql --max_allowed_packet=32M
That sets the packet size to 32MB.
The server's default max_allowed_packet value
is 1MB. You can increase this if the server needs to handle big
queries (for example, if you are working with big
BLOB columns). For example, to set the
variable to 16MB, start the server like this:
mysql> mysqld --max_allowed_packet=16M
You can also use an option file to set
max_allowed_packet. For example, to set the
size for the server to 16MB, add the following lines in an
option file:
[mysqld]
max_allowed_packet=16M
It is safe to increase the value of this variable because the
extra memory is allocated only when needed. For example,
mysqld allocates more memory only when you
issue a long query or when mysqld must return
a large result row. The small default value of the variable is a
precaution to catch incorrect packets between the client and
server and also to ensure that you do not run out of memory by
using large packets accidentally.
You can also get strange problems with large packets if you are
using large BLOB values but have not given
mysqld access to enough memory to handle the
query. If you suspect this is the case, try adding
ulimit -d 256000 to the beginning of the
mysqld_safe script and restarting
mysqld.
