This topic describes the default read weights allocated to RDS instances of various specifications.

Weight values list

When the read weights are automatically allocated for instances by the system, the values of these weights are fixed, as shown in the following table.

Table 1. Weights for RDS MySQL read-only instances
Specification code Specification type Memory CPU Weight
rds.mysql.t1.small General-purpose instance 1 GB 1 100
rds.mysql.s1.small General-purpose instance 2 GB 1 100
rds.mysql.s2.large General-purpose instance 4 GB 2 200
rds.mysql.s2.xlarge General-purpose instance 8 GB 2 200
rds.mysql.s3.large General-purpose instance 8 GB 4 400
rds.mysql.m1.medium General-purpose instance 16 GB 4 400
rds.mysql.c1.large General-purpose instance 16 GB 8 800
rds.mysql.c1.xlarge General-purpose instance 32 GB 8 800
rds.mysql.c2.xlarge General-purpose instance 64 GB 16 1600
rds.mysql.c2.xlp2 General-purpose instance 96 GB 16 1600
rds.mysql.c2.2xlarge General-purpose instance 128 GB 16 1600
mysqlro.x8.medium. 1 Dedicated instance 16 GB 2 200
mysqlro.x4.large. 1 Dedicated instance 16 GB 4 200
mysqlro.x8.large. 1 Dedicated instance 32 GB 4 400
mysqlro.x4.xlarge. 1 Dedicated instance 32 GB 8 400
mysqlro.x8.xlarge. 1 Dedicated instance 64 GB 8 800
mysqlro.x4.2xlarge. 1 Dedicated instance 64 GB 16 800
mysqlro.x8.2xlarge. 1 Dedicated instance 128 GB 16 1600
mysqlro.x4.4xlarge. 1 Dedicated instance 128 GB 32 1600
rds.mysql.st.d13 Dedicated-host instance 220 GB 30 3000
rds.mysql.st.h43 Dedicated-host instance 470 GB 60 6000

Specify whether a SQL statement is sent to the master instance or a read-only instance by adding a hint

In addition to the weight distribution system of read/write splitting, hints serve as a complementary SQL syntax to specify whether a SQL statement is executed on the master instance or a read-only instance.

Hints supported by RDS read/write splitting are as follows:

  • /*FORCE_MASTER*/: specifies that a SQL statement is executed on the master instance.
  • /*FORCE_SLAVE*/: specifies that a SQL statement is executed on a read-only instance.
    Note When you use the /*FORCE_MASTER*/ hint in an SQL statement, the SQL statement is routed to the Primary RDS instance even if the read weight of the master instance is 0.

For example, after a hint is prefixed to the following statement, the statement is always routed to and executed on the master instance regardless of the preset weight: 

/*FORCE_MASTER*/ SELECT * FROM table_name;