This topic provides answers to some frequently asked questions (FAQ) about async replication and continuous data replication (CDR) for Elastic Compute Service (ECS) disaster recovery.
What instance types does async replication for ECS disaster recovery support? Are there limits on cloud disks or IP addresses?
Limitations apply to available regions and zones, cloud disk specifications, ECS network interface controllers (NICs), and configuration quotas.
Most instance types are supported.
Only Enterprise SSDs (ESSDs) are supported. ESSD Entry or ESSD AutoPL disks are not supported. For more information, see Limits.
ECS networks have the following limits:
A single network interface controller (NIC) has the following limitations:
For some operating systems, the ENI at the disaster recovery site is not automatically configured after a failover. After a failover, you must check and configure the ENI at the disaster recovery site to ensure that the network works as expected. For more information, see Configure a secondary ENI.
Multiple network interface controllers (NICs) are subject to the following limitations:
After an ECS instance is attached to a secondary ENI, some images cannot automatically detect the IP address of the secondary ENI or add a route. As a result, the secondary ENI cannot work as expected.
If an ECS instance is configured with a secondary ENI, you must check the IP address of the secondary ENI after a failover to ensure that the secondary ENI works as expected. For more information, see Configure a secondary ENI.
ENIs or ECS instances that are outside the VPC of the site pair are not supported.
Where can I change the IP address of the disaster recovery site for async replication for ECS disaster recovery?
On the Network Information tab of the instance details page, you can manually assign an IP address to the disaster recovery site.
In the Preview Basic Information panel that appears when you confirm the replication, you can manually specify an IP address for the disaster recovery site.
Does async replication for ECS disaster recovery support configuration changes at the disaster recovery site?
During the initialization phase of a protection group, configuration changes such as adding a new disk can be automatically synchronized to the disaster recovery site, provided that the number and capacity of disks in the protection group do not exceed the limits.
If a protection group is in a state such as replicating or switching over, configuration changes at the production or disaster recovery site are not supported. Such changes can interfere with the disaster recovery switchover and recovery processes. The resources of the protection group at both sites are checked, and an alert is generated if an exception is detected. Evaluate the risks and proceed with caution.
What do I do if I cannot select an instance when adding instances for async replication for ECS disaster recovery?
This issue may occur because async replication for disaster recovery is subject to limits on regions, zones, disk types, networks, and configuration quotas. You can troubleshoot the issue based on the prompts in the console. For more information, see Limits.
What do I do if the console reports an abnormal instance type at the disaster recovery site when I start replication for async replication for ECS disaster recovery?
This issue occurs because the instance type of the protected instance is unavailable or out of stock at the disaster recovery site. Follow the prompts on the interface to change the instance type to a supported one using the Modify Instance Type option. If the operating system or IP address is reported as abnormal, you can use the Modify Operating System and Modify DR Site IP options to resolve the operating system and IP address conflict issues.
What do I do if a failover fails for async replication for ECS disaster recovery?
The protection group is in the Failover Failed state.
In the console, follow the prompts to click the ID of the failed task. On the historical tasks tab, you can view the detailed cause of the error.
For example, Not have any stock of instance type family ... indicates that the instance type family is out of stock. On the Protected Instances page, use the Modify DR Site Instance Type option and then retry the operation.
What are the differences between continuous data replication (CDR) and async replication for ECS disaster recovery?
Async replication is a feature that uses the data replication capability of block storage to protect data across regions or zones. For more information, see How async replication works.
The following table describes the differences between async replication and continuous data replication (CDR).
Item | Continuous data replication (CDR) | Async replication for disaster recovery |
Main scenarios | Disaster recovery for a single virtual machine (VM). Suitable for customers who do not mind system intrusion. | Disaster recovery that ensures VM group consistency. Suitable for customers who do not want system intrusion. |
System intrusiveness | Yes | No |
Replication implementation | An agent is installed on the protected instance to embed into the OS. The agent replicates data written to the disks in real time and sends the data to a gateway. The gateway stores the data in an Object Storage Service (OSS) bucket and then writes the data to the disk at the disaster recovery site. | Data is replicated using mechanisms such as async replication and snapshots of block storage. |
Recovery implementation | Supports multiple recovery points. At the secondary site, a shadow ECS instance and a gateway server are created for the protected instance. Data is pulled from the data transfer station and written through the shadow ECS instance. Then, recovery points are created based on the snapshot mechanism. | Supports only a single recovery point. A recovery point is generated by replicating a snapshot to the secondary site. |
Consistency group | Not supported | Support |
What are the RPO and RTO of CDR for ECS disaster recovery?
Core business data from on-premises data centers is replicated to the cloud in real time. This achieves an RPO of seconds to minutes. If a major failure occurs in an on-premises data center, services can be restored in the cloud within minutes. This achieves an RTO in minutes.
Which operating systems does CDR for ECS disaster recovery support?
CDR supports mainstream Windows and Linux operating systems. For more information, see Operating systems.
The following table describes the operating systems that support ECS disaster recovery (CDR).
Only the operating systems listed in the following table are supported. For other operating systems, we recommend that you use the async replication feature.
Operating system | Version |
Windows Server | 2008 R2, 2012, 2012 R2, and 2016 |
Linux | Important For Linux systems, the /boot partition and the / partition must be on the same disk. If they are not, resize the disk manually before you register the protected instance for CDR.
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What are the snapshot retention policies for CDR for ECS disaster recovery?
The recovery point feature of ECS Disaster Recovery uses the snapshot feature of shadow disks to ensure that protected servers can be restored to a specific historical version.
The snapshot retention policies are as follows:
If a recovery point has been used for disaster recovery drills or a switchover, it is not subject to the following rules.
All recovery points from the last day are retained.
For example, if the current UTC time is 17:00:00 on October 12, 2020, 'the last day' refers to the 41-hour period from 00:00:00 on October 11, 2020 to 17:00:00 on October 12, 2020.
For the last week, the last recovery point of each day is retained.
For the last month, the last recovery point of each week is retained.
Recovery points older than one month are not retained.
Does CDR for ECS disaster recovery support resizing or adding disks to a source ECS instance?
Currently, you can resize or add disks only to a source ECS instance that runs Linux and is part of a cross-region or cross-zone site pair for cloud disaster recovery.
After you resize a disk online or add a new disk to the source ECS instance, ECS Disaster Recovery detects the disk changes within 5 minutes. ECS Disaster Recovery stops the current server replication, automatically adjusts the disk capacity of the destination shadow instance, and then repairs the replication. After the repair is complete, real-time replication resumes. This process is completed automatically and requires no manual intervention. The time required for this process depends on the disk size. In the console, you can observe the status change from 'repairing replication' to 'replicating'.
ECS Disaster Recovery does not support scaling in or removing disks from the source ECS instance. These operations may cause replication errors or data loss.