Async replication is a feature that protects data across regions or across zones within the same region based on the data replication capability of Elastic Block Storage (EBS). This feature allows data to be asynchronously replicated between disks across regions or across zones within the same region to implement disaster recovery. You can use the feature to implement disaster recovery for critical business to protect data in databases and improve business continuity.
Scenarios
The async replication feature is suitable for users that require high data security and want to implement disaster recovery and cross-region migration of business data.
Disaster recovery scenario
If the primary disk in a replication pair fails, you can use the failover feature to switch the primary and secondary disks over. During the switchover, the failover feature disconnects the original replication link and fails services over to the disaster recovery system by attaching the new primary disk (original secondary disk) to an Elastic Compute Service (ECS) instance that is used for disaster recovery.
Cross-region migration scenario
If you want to migrate your business data across regions, you can use the reverse replication sub-feature, instead of the image or snapshot replication feature.
Feature description
How the async replication feature works
The async replication feature allows data to be asynchronously replicated from an enhanced SSD (ESSD) (primary disk) to another ESSD (secondary disk) that has the same specifications. The primary and secondary disks can reside in different zones within the same region or reside in different regions. If the primary disk fails, you can fail over to the secondary disk. After the primary disk recovers, you can restore data from the secondary disk. The following figures show how the async replication feature works when the primary and secondary disks reside in different zones within the same region or reside in different regions.
The primary and secondary disks reside in different regions
The primary and secondary disks reside in different zones within the same region
Operations
The following table describes the operations supported by the async replication feature.
Operation | Description | References |
Create a replication pair | Before you can use the async replication feature to implement disaster recovery across regions or across zones within the same region, you must create a replication pair. | |
Enable the async replication feature | After you create a replication pair, you must enable the async replication feature for the pair to replicate data from the primary disk to the secondary disk across regions or across zones within the same region on a periodic basis. | |
Implement disaster recovery | After you create and activate a replication pair, if the primary disk fails, you can use the failover and reverse replication sub-features of async replication to implement disaster recovery. | |
Delete a replication pair | The functionality of disks in a replication pair is limited. If an existing replication pair is no longer required to implement disaster recovery across regions or across zones in the same region or needs to be replaced, you can delete the replication pair. |
Terms
The following table describes the terms related to the async replication feature.
Term | Description |
asynchronous replication | The async replication feature supports only asynchronous replication. Unlike synchronous replication, asynchronous replication replicates data from a disk to another disk across regions or across zones within the same region on a periodic basis. In asynchronous replication, the data on the source disk may be inconsistent with that on the destination disk. |
primary site | A primary data center that can independently run to support the normal operation of business. |
secondary site | The data center that serves as a disaster recovery site for a primary site. If the primary site fails, the secondary site takes over business to ensure business continuity. |
primary disk | The disk from which data is replicated to implement disaster recovery. The primary disk is also called the source disk. After the replication relationship of a replication pair is reversed, the primary disk is converted to the secondary disk. |
secondary disk | The disk to which data is replicated. The secondary disk is also called the destination disk. After the replication relationship of a replication pair is reversed, the secondary disk is converted to the primary disk. |
recovery point objective (RPO) | The amount of data that may be lost due to a disk exception. The RPO is measured in time and is used as a data metric. The default value of RPO is 15 minutes in async replication. This value indicates that data written to a primary disk from up to 15 minutes prior may be lost when an exception occurs on the disk. |
recovery time objective (RTO) | The duration for which a primary disk recovers after an exception occurs on the disk. RTO is used as a data metric in async replication. For example, if the value of RTO is 1 hour, the data of a primary disk can be restored and run as expected within 1 hour after an exception occurs on the primary disk. |
replication pair | The replication relationship that is established between a primary disk, a secondary disk, and the configurations for asynchronous replication. |
failover | A sub-feature of async replication that allows you to enable read and write permissions on the secondary disk and fail over to the secondary disk when the primary disk fails. |
reverse replication | A sub-feature of async replication that can reverse replication relationships to replicate data from secondary disks to primary disks. |
Billing
Async replication supports the following billing methods:
Subscription: You are charged for bandwidth. You can select 10 Mbit/s, 20 Mbit/s, 50 Mbit/s, and 100 Mbit/s of bandwidth.
Pay-as-you-go: You are charged based on the amount of replicated data.
NoteYou are charged only for cross-region replication on a pay-as-you-go basis. You are not charged for cross-zone replication in the same region.
Limits
Limits on regions
The async replication feature is supported in the following regions and zones:
China (Hangzhou): Zone G, Zone H, Zone I, and Zone K
China (Shanghai): Zone B, Zone E, Zone F, Zone G, and Zone L
China (Beijing): Zone F, Zone G, Zone H, and Zone J
China (Zhangjiakou): Zone A
China (Shenzhen): Zone D and Zone E
China (Heyuan): Zone A and Zone B
China (Chengdu): Zone A and Zone B
China (Hong Kong): Zone B and Zone C
Singapore: Zone B and Zone C
Indonesia (Jakarta): Zone A and Zone B
US (Silicon Valley): Zone A and Zone B
US (Virginia): Zone A and Zone B
SAU (Riyadh): Zone A and Zone B
China East 2 Finance: Zone G and Zone K
China North 2 Finance (Private Preview): Zone L and Zone K
Limits on instance types
The following table describes the limits that apply to the async replication feature.
Item | Description |
Replication pairs that can be created for a single disk | 1 |
Replication cycle | 15 minutes (Data is asynchronously replicated from a primary disk to a secondary disk every 15 minutes.) |
Replication rate | The replication rate can be up to 100 MB/s and varies based on the system load. |
Primary disk category | A primary disk must be an ESSD. |
Secondary disk category | A secondary disk must be of the same disk category and have the same performance level and capacity as the associated primary disk. |
Limits on disk performance
The limits described in the following table apply to primary and secondary disks when you use the async replication feature.
1: After a replication pair is activated, the secondary disk enters the read-only state and no users have write permissions on the disk.
2: Due to RPO, the data of a snapshot created for a primary disk may be inconsistent with that of a snapshot created at the same time for the associated secondary disk.
3: Replication is restricted to encrypted disks. Cross-replication between encrypted disks and unencrypted disks is not supported.
Item | Supported by the primary disk | Supported by the secondary disk |
Read and write operations | √ | × 1 |
Disk deletion | × | × |
Disk initialization | × | × |
Disk resizing | × | × |
Disk attaching | ✓ | × |
Snapshot creation | ✓ | ✓ 2 |
Rollback based on snapshots | ✓ | × |
Disk category change | × | × |
Performance level change | × | × |
Disk encryption | ✓ | ✓ 3 |
Multi-attach | × | × |
Disk migration together with instances | × | × |