ApsaraDB RDS:Read-only instance types

RDS lets you scale out read performance by adding read-only instances.

For more information about primary instance types, see Primary instance types.

Read-only instance types for each database engine

• RDS for MySQL read-only instance types (Standard Edition, formerly X86)

• RDS for MySQL read-only instance types (Yitian Edition, formerly ARM)

• RDS for PostgreSQL read-only instance types

• RDS for SQL Server read-only instance types

Instance families

The instance family determines how resources are allocated and directly affects instance performance.

IOPS

Input/output operations per second (IOPS) measures a storage system's ability to handle read and write requests. It is a key metric for random I/O performance. The main factors that affect an instance's IOPS are its instance type, storage class, and storage capacity. You can find the maximum IOPS for an instance type in the read-only instance type tables for each engine. The actual maximum IOPS of an instance is calculated as follows:

• Instances with Premium Local SSDs: The actual maximum IOPS is determined solely by the instance type. The value listed in the primary instance type table is the actual maximum IOPS for the instance.

• Instances that use cloud disks: The actual maximum IOPS is determined by the instance type, storage capacity, and storage class. The formula is as follows:

  Storage class		Formula for actual maximum IOPS (Storage capacity unit: GB)
  Premium performance disk	Performance burst enabled	min{Maximum IOPS of the instance type, 1,000,000}
  	Performance burst disabled	min{Maximum IOPS of the instance type, 1,800 + 50 * Storage capacity, 50,000}
  Enterprise SSD (ESSD)	PL3	min{Maximum IOPS of the instance type, 1,800 + 50 * Storage capacity, 1,000,000}
  	PL2	min{Maximum IOPS of the instance type, 1,800 + 50 * Storage capacity, 100,000}
  	PL1	min{Maximum IOPS of the instance type, 1,800 + 50 * Storage capacity, 50,000}
  Standard SSD		min{Maximum IOPS of the instance type, 1,800 + 30 * Storage capacity, 25,000}

Example: Calculate the actual IOPS for an instance that uses a PL1 ESSD, has an instance type of mysql.x2.large.2c, and has a storage capacity of 20 GB.

The actual maximum IOPS for this instance is 2,800, which is the minimum of these three values. The performance is mainly limited by the storage capacity.

I/O bandwidth (throughput)

I/O bandwidth reflects the ability of a storage system to continuously read and write data. It is a key metric for sequential I/O performance. The main factors that affect an instance's I/O bandwidth are its instance type, storage class, and storage capacity. You can find the maximum I/O bandwidth for an instance type in the read-only instance type tables for each engine. The actual maximum I/O bandwidth of an instance is calculated as follows:

Example: Calculate the actual maximum I/O bandwidth for an instance that uses a PL3 ESSD, has an instance type of mysql.x2.large.2c, and has a storage capacity of 5,000 GB.

The actual maximum I/O bandwidth for this instance is the minimum of these three values: 192. This is mainly limited by the instance type.

Relationship between IOPS and I/O bandwidth

A disk's IOPS and I/O bandwidth are not independent performance metrics. In addition to being dependent on the instance type, storage class, and storage capacity, they also constrain each other:

• Conversion formula: I/O bandwidth (MB/s) = IOPS × I/O block size (KB) / 1,024. In this formula, I/O block size refers to the size of a single disk I/O operation (4 KB for cloud disks by default), not the database page size.

• Constraint: In most cases, IOPS and I/O bandwidth do not reach their maximum values at the same time.

  • When IOPS reaches its maximum value, if the I/O block size is small (such as 4 KB), the instance's I/O bandwidth may not reach its maximum value. In this case, IOPS becomes the performance bottleneck.

  • When I/O bandwidth reaches its maximum value, if the I/O block size is large (such as 256 KB), the instance's IOPS may not reach its maximum value. In this case, I/O bandwidth becomes the performance bottleneck.

Read-only instance pricing

For the prices of read-only instances, see the official purchase page.