ApsaraDB RDS:Create a SQL Server read-only instance

Prerequisites

You must create a SQL Server read-only instance under a primary instance that meets all the following conditions:

• Instance version: SQL Server 2017 Enterprise Cluster Edition, 2019 Enterprise Cluster Edition, or 2022 Enterprise Cluster Edition

• Instance edition: Cluster Edition

• Storage class: enterprise SSD (ESSD) or Premium ESSD

• Billing method: subscription or pay-as-you-go (does not support Serverless instances)

Important notes

• You can only create read-only instances under a primary instance. You cannot convert an existing primary instance into a read-only instance.

• Creating a read-only instance copies data from the secondary instance, so it does not affect the primary instance.

• Number of read-only RDS instances: You can create up to seven read-only RDS instances for a primary RDS instance.

• Instance backup: You cannot configure backup policies or manually create backups for read-only RDS instances. These are configured and created on the primary RDS instance. You cannot create temporary RDS instances from backup files or any point in time. You cannot overwrite RDS instances by using backup sets. After a read-only RDS instance is created, you cannot use backup sets to overwrite the primary RDS instance to restore data.

• Data migration: You cannot migrate data to read-only RDS instances.

• Database management: You cannot create or delete databases on read-only RDS instances.

• Account management: You cannot create or delete accounts, grant permissions to accounts, or change the passwords of accounts on read-only RDS instances.

• Instance creation time: Read-only instance creation time increases with the primary instance’s data volume and I/O performance. Before creating a read-only instance, evaluate and optimize the primary instance’s data volume and I/O performance to reduce creation time.

• Storage space: The storage space of a read-only instance must be no smaller than that of the primary instance.

Billing rules

You are charged for the read-only RDS instances that you create based on the subscription billing method or the pay-as-you-go billing method. For more information, see Read-only instance types.

Create a read-only instance

Create a read-only instance using the RDS console or the API (CreateReadOnlyDBInstance).

1. Log on to the ApsaraDB RDS console and go to the Instances page. In the top navigation bar, select the region in which the RDS instance resides. Then, find the RDS instance and click the instance ID.

2. In the Instance Distribution section, click Read-only Instance, then click add on the right.

3. Configure basic resources, including billing method, storage class, and zone. Click Next: Instance Configuration.

   1. Select a Billing Method.

      Billing method	Recommendation	Advantage
      Subscription	For long-term RDS usage, select Subscription (one-time payment). Then select a Duration in the bottom-left corner.	Subscription costs less than pay-as-you-go, and longer durations offer greater discounts.
      Pay-As-You-Go	For short-term RDS usage, select Pay-As-You-Go (hourly billing). You can create a Pay-As-You-Go instance first, then switch to subscription after confirming it meets your needs.	You can release the instance anytime to stop billing.

   2. Select a database Storage Type.

      For supported storage classes, see Storage classes.

   3. Select a Zone for the instance.

      Note

      • A zone is an independent physical area within a region. Zones in the same region have no practical differences.

      • ECS instances achieve slightly better performance accessing RDS instances in the same zone versus other zones in the same region, though the difference is small.

   4. Select an Instance Type.

      Choose a category (Dedicated or General-purpose) and a specific specification (CPU cores, memory, maximum connections, etc.).

      Category	Description	Features
      General-purpose	Dedicated: memory and I/O Shared: CPU and storage	Low cost with high value.
      Dedicated	Dedicated: CPU, memory, storage, and I/O Note Exclusive is the highest tier of Dedicated instances, occupying an entire server’s CPU, memory, storage, and I/O.	Better and more stable performance.

      Note

      • Each instance type has corresponding CPU cores, memory, maximum connections, and maximum IOPS.

      • To ensure sufficient I/O performance for data synchronization, make the read-only instance type (memory) at least as large as the primary instance.

   5. Select Capacity.

      Storage space includes data, system files, log files, and transaction files. The available range (minimum and maximum) depends on your selected instance type and storage class. Adjust storage space in increments of at least 5 GB.

      Note

      • Read-only instance storage space must be no smaller than the primary instance.

      • For some local SSDs, storage space is tied to the instance type. ESSD and standard SSD are not subject to this restriction.

4. Configure instance resources, including network type, vSwitch, and resource group. Click Next: Confirm Order.

   1. Network Type is fixed as VPC.

   2. Select a VPC and VSwitch of Primary Node.

      We recommend using the same VPC as your ECS instance. ECS and RDS instances in different VPCs cannot connect over the internal network.

      Note

      ECS and RDS instances can connect over the internal network if they share the same VPC, even with different vSwitches.

   3. Set additional custom parameters. Keep default values unless you have specific requirements.

      Parameter	Description
      Release Protection	When Billing Method is Pay-As-You-Go, you can enable instance release protection to prevent accidental release.
      Resource Group	Select the default or a custom resource group for easier instance management.
      Instance Name	Customize the name. Use 2–255 characters. Start with a letter or Chinese character. You can include numbers, underscores (_), or hyphens (-).
      Character Set Collation Rule	Defaults to the primary instance’s system collation.

5. Confirm Parameter Configuration, Quantity, and Duration (subscription instances only). Click Pay Now and complete payment.

   Note

   • For subscription instances, we recommend selecting Auto-renewal to avoid service interruption from missed renewals.

   • Instance creation takes 1–10 minutes. Refresh the page to check status.

View read-only instances

You can view read-only instance details on the Instances page, the Cluster Management page of the target instance, or the Basic Information page of the target instance.

Next steps

After creating a read-only instance, enable read/write splitting and configure read weights. The system will route read requests based on your configured weight ratios.

FAQ

SELECT
    ag.name AS [availability_group_name]
    , d.name AS [database_name]
    , ar.replica_server_name AS [replica_instance_name]
    , drs.truncation_lsn
    , drs.log_send_queue_size
    , drs.redo_queue_size
FROM
    sys.availability_groups ag
    INNER JOIN sys.availability_replicas ar
        ON ar.group_id = ag.group_id
    INNER JOIN sys.dm_hadr_database_replica_states drs
        ON drs.replica_id = ar.replica_id
    INNER JOIN sys.databases d
        ON d.database_id = drs.database_id
WHERE drs.is_local=0
ORDER BY
    ag.name ASC, d.name ASC, drs.truncation_lsn ASC, ar.replica_server_name ASC