Limits
This topic describes the limits of Continuous Data Replication (CDR) for operating systems, infrastructure, databases, and applications.
Operating system
CDR support is currently limited to the following server versions.
Operating system | Version |
Windows Server | 2008 R2, 2012, 2012 R2, 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 before you register the protected instance for ECS disaster recovery.
|
Infrastructure
CDR performs disk-level data replication at the operating system layer, making it independent of the underlying infrastructure. The following platforms have been tested by Alibaba Cloud.
Infrastructure | Version |
Physical server | Fully supported |
vSphere | 5.5, 6.0, 6.5, 6.7, and 7.0 |
Databases and applications
CDR uses application-agnostic replication technology and supports all types of databases and applications.
Many applications require automation scripts to ensure application consistency. The CDR service provides efficient tools that integrate with various scripts to ensure application recovery.
Other limits
The following limits also apply to CDR:
Disaster recovery operations cannot be performed on ECS instances that use the NVMe protocol to communicate with cloud disks.
Due to Alibaba Cloud system disk limits, a server cannot be fully recovered on an ECS instance if its system disk is a single physical volume that exceeds 2 TB.
A single physical volume of a data disk cannot exceed 32 TB.
Disk write limits:
Linux system
For an average I/O size of 4 KB, the maximum supported disk write speed is approximately 10 MB/s. For an average I/O size of 64 KB, the maximum supported disk write speed is approximately 30 MB/s.
Windows system
The maximum supported disk write speed is 10 MB/s.
The total supported throughput for a single site is 400 Mb/s.
If this throughput is exceeded, the Recovery Point Objective (RPO) and Recovery Time Objective (RTO) may increase. When you plan for disaster recovery, fully assess the workload of the protected servers. Consider the disk write volume as an important factor in your disaster recovery planning.
The maximum internal bandwidth supported by a replica ECS instance is 80 Mb/s.
The default replica ECS instance supports a maximum internal bandwidth of 80 Mb/s. If the continuous replication rate exceeds 80 Mb/s, data processing at the disaster recovery site may be delayed, which increases the RTO. To increase the internal bandwidth of the replica ECS instance, you can upgrade the ECS instance to a suitable instance type. For more information, see ECS instance families.
The Aliyun Replication Service (AReS) process does not support unexpected termination. If the process terminates unexpectedly, replication fails.
Protected servers do not support restarts from a powered-off state, such as forced shutdowns or ECS instance failures. This type of restart causes replication to fail.
Protected servers do not support disk scale-in operations. Windows systems do not support disk scale-out operations. These operations cause replication to fail.
If replication fails on a Windows system, it cannot be repaired. You must restart the full replication process.
References
Async replication for cloud disks is a feature used by the Hybrid Disaster Recovery service to provide cross-region or cross-zone data protection. This feature replicates data at the block storage level and does not require an agent to be installed on the protected instance.
The following table compares disaster recovery using asynchronous replication with Continuous Data Replication (CDR).
Comparison Criterion | Continuous Data Replication (CDR) | Disaster recovery using async replication |
Primary supported scenarios | Suitable for disaster recovery of a single virtual machine. Ideal for customers who do not mind system intrusion. | Suitable for disaster recovery with VM group consistency. Ideal for customers who want an agentless solution. |
System intrusion | Yes | No |
Replication method | An agent is installed on the protected instance and embedded in the OS. The agent replicates data written to the disk in real time and sends it to a gateway. The gateway stores the data in OSS, which acts as a transit station, and then writes the data to the disks at the disaster recovery site. | Data replication is achieved through mechanisms such as async replication and snapshots for Elastic Block Storage disks. |
Implementing recovery | Supports multiple recovery points. The secondary site creates a shadow ECS instance and a gateway instance for the protected instance. It pulls data from the data transit station and writes it through the shadow ECS. Recovery points are then created based on snapshots. | Supports only a single recovery point. A recovery point is generated by replicating a snapshot to the secondary site. |
Consistency protection group | Not supported | Supported |
We recommend trying this feature. For more information, see Async Replication for Disaster Recovery Overview.