A communication packet is a single SQL statement sent to the MySQL server, a single row that is sent to the client, or a binary log event sent from a replication source server to a replica.
The largest possible packet that can be transmitted to or from a MySQL 8.4 server or client is 1GB.
When a MySQL client or the mysqld server
receives a packet bigger than
max_allowed_packet
bytes, it
issues an
ER_NET_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 --max_allowed_packet=32M
That sets the packet size to 32MB.
The server's default
max_allowed_packet
value is
64MB. 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 128MB, start the server like this:
$> mysqld --max_allowed_packet=128M
You can also use an option file to set
max_allowed_packet
. For
example, to set the size for the server to 128MB, add the
following lines in an option file:
[mysqld]
max_allowed_packet=128M
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.