Microsoft Storage Replica is a technology introduced starting with Windows Server 2016 which among the main usage scenarios involves volume replication, synchronously or asynchronously, between servers or clusters, for disaster recovery purposes. Nowadays, several Microsoft customers continue to use DFS Replication (DFS-r) as a Disaster Recovery solution for unstructured data such as home folders and departmental shares. The question many are asking is whether the recent Storage Replica technology really takes the place of the well-established DFS-r mechanism. In this article the characteristics of the two solutions will be explored in order to clarify when it is convenient to use Storage Replica and when DFS Replication (DFS-r).
DFS Replication (DFS-r)
DFS Replication is a solution that can be activated through a role in Windows Server that allows you to replicate folders on different servers and geographic sites. The solution is based on an efficient replication engine, that contemplates the presence of more masters, which can be used to keep folders synchronized between different servers, even through network connections with limited bandwidth. DFS-r uses a compression algorithm known as Remote Differential Compression (RDC), able to detect changes in a file and replicate only the changed blocks instead of the entire file. DFS-r has long since replaced the File Replication Service (FRS) and it is also used for Active Directory Domain Services SYSVOL folder replication (AD DS) in domains where the functional level is at least Windows Server 2008.
The activation of DFS-r involves the creation of replication groups that contain the files and folders to be replicated:
Figure 1 – DFS-r Replication Process
For more information about the DFS Replication service (DFS-r) you can see the related Microsoft documentation.
Storage Replica
Storage Replica is a technology introduced from Windows Server 2016 and it allows the replication of volumes between servers or clusters for Disaster Recovery purposes.
Figure 2 – Server-to-server and Cluster-to-cluster storage replication scenarios
This technology also allows you to create stretch failover cluster with nodes spread over two different site, keeping storage in sync.
Figure 3 – Stretch clustered storage replication scenarios
In Windows Server 2016 the ability to use storage replication is only available if you use the Datacenter Edition of the operating system, while in Windows Server 2019 there is the possibility to activate Storage Replica also by adopting the Standard Edition, but with the following limitations:
- You can replicate a single volume instead of an unlimited number of volumes.
- The maximum size of the replicated volume should not exceed 2 TB.
For more information on Storage Replica, please consult the related Microsoft documentation.
Comparison of the solutions
The DFS-r solution is particularly effective in environments with limited network bandwidth and where it is necessary to replicate content on different nodes for which a limited number of changes are expected. However, DFS-r has significant limitations as a data replication solution, including:
- Cannot replicate files in use or open.
- It does not have a synchronous replication mechanism.
- The latency for asynchronous replication can be of considerable duration (minutes, hours or even days).
- It is based on a database that may require costly consistency checks as a result of any system outages.
- The burden of environmental management is high.
The Storage Replica solution does not have the limitations listed above, but it is good to take into consideration the following aspects that differentiate it from the DFS-r solution and that in some scenarios could be considered as critical elements:
- Replication occurs at the volume level and only one-to-one replication between volumes is allowed. However, it is possible to replicate different volumes between multiple servers.
- Allows you to replicate synchronously or asynchronously, but it is not designed for low-bandwidth, high-latency networks.
- Users are not allowed to access protected data on the target server while replication is in progress.
- To validate the effectiveness of the replication process, it is still possible to carry out a Failover Test, which allows you to mount a writable snapshot of the replicated storage. To perform this operation, for testing purposes or backup, you must have a volume, not involved in replication, on the destination server. The Failover Test has no impact on the replication process, which will continue to ensure the protection of the data and the changes to the snapshot will remain circumscribed to the test volume.
How to replace DFS Replication (DFS-r) with Storage Replica?
If the characteristics of Storage Replica are not considered blocking, this latest technology can be adopted to replace the DFS Replication solution (DFS-r). The high-level process involves the following steps:
- Install new systems at least Windows Server 2016, paying attention to evaluate the limits imposed by the Standard Edition, and configure storage. To learn more about improvements in Storage Replication with Windows Server 2019 you can consult this article.
- Migrate the data that you want to replicate to one or more volumes of data (for example through Robocopy).
- Enable Storage Replica replication and complete initial synchronization.
- We recommend enabling snapshots through Volume Shadow Copies, in particular in the case of asynchronous replication. Snapshots are also replicated along with the files. In case of emergency, this will allow you to restore files from snapshots on the target server that may have only been partially replicated asynchronously.
- Share the data on the source server and make it accessible through a DFS namespace. This aspect is important to ensure that users can still access the data when the server name changes during the activation of the Disaster Recovery plan. On the replication target server (DR site) you can create shares (not accessible during normal operations). The server on the DR site can be added to the DFS Namespaces keeping the target folders disabled.
If disaster recovery scenario needs to be activated, using the storage replica solution, you should do the following:
- Make the server located on the DR site primary, so that it can show replicated volumes to users.
- In case of synchronous replication, no data recovery will be required, unless during the loss of the origin server the user was using an application that wrote data without transaction protection.
- In case of asynchronous replication, you may need to mount a snapshot to ensure application-wide data consistency.
- Enable the target folders in DFS Namespaces to allow users to access their data.
Conclusions
Microsoft is continuing to make major investments in storage and Storage Replica is the tangible result that allows customers to adopt an effective and performing storage replication solution. In the Disaster Recovery field, there are several scenarios where Storage Replica can replace the DFS Replication service (DFS-r), however you should carefully evaluate the characteristics of both solutions to choose the one that best suits your usage scenario.
