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If there is a common theme in making sure your data is resilient to hardware failure and is highly available, your data should exist in multiple places. Placing all your data in a single location or a single set of hardware is never a good idea. You can see an example of this in the 3-2-1 backup best practice rule. You should have at least three copies of your data stored on two different kinds of media, with at least one copy stored offsite. Additionally, you want to have your data as close to your end users as possible.
Microsoft Windows Server technology includes various ways to replicate your data. One is Distributed File System (DFS) Replication, and another is the newer Storage Replica technology. What are the advantages and disadvantages of each? When would you choose one over the other? Let's take a look at Storage Replica and DFS Replication and explore this topic further.
DFS Replication ^
DFS Replication (DFSR) has been around for quite some time. The first release of DFS Replication was introduced in Windows Server 2003 R2. It replaced File Replication Services (FRS). What is DFSR?
DFSR is a role service that allows replicating folders, including DFS namespace paths, to multiple servers across sites. It is a replication technology that leverages multi-master replication. If you haven't set up DFSR replication for use with file replication with a DFS namespace, you may be familiar with it in the context of Active Directory. The underlying ADDS replication technology is DFSR. DFSR has many features that enhance the replication features, including differential compression. Keeping folders synchronized across multiple servers and sites helps increase data availability. It gives users the ability to access files and folders in different locations via the server that is nearest to them. After years of production implementation, DFSR technology has been proven to replicate data across multiple site topologies effectively.
To use DFS Replication, create replication groups, and then add the folders you want to replicate to the groups.
When you create replicated groups containing member servers, the folders do not have to reside on the corresponding volumes on each server. They can be different for each server. Also, replicated folders do not have to be part of a DFS namespace.
Now let's look at a different type of replication technology called Storage Replica.
Storage Replica ^
Storage Replica technology is a newer Windows Server technology that replicates entire volumes between servers or server clusters for disaster recovery purposes. It was introduced with Windows Server 2016 and is an installable feature by the same name in Windows Server 2019. Storage Replica provides the means of disaster recovery in Windows Server. The ability to replicate data synchronously between failure domains is a great way to ensure that data is protected against a site failure. It also provides the ability to switch proactively between primary and secondary sites. It allows shifting workloads in preparation for a maintenance period or even an oncoming foreseeable disaster when there is just a few moments' notice.
Storage Replica supports two well-known replication configurations, including:
- Synchronous replication—With synchronous replication, data is written on both the primary volume and the secondary volume at the same time. With synchronous replication, you ensure there is no skew of data between the primary and secondary disaster recovery locations. Storage Replica synchronous replication creates a mirror of the primary volume on the secondary volume. It requires an extremely low-latency network.
- Asynchronous replication—With asynchronous replication, the data is first committed to the primary storage volume before the data write occurs in the secondary location. In this replication configuration, Storage Replica can tolerate higher network latencies and replicate data between metropolitan sites. There is no guarantee that the secondary site will have identical copies of data in the asynchronous replication configuration when there is a failure.
Storage Replica or DFS Storage Replication ^
Given that Storage Replica is the newer of the two technologies, is it the preferred choice in all situations or configurations? Not necessarily. While Microsoft touts Storage Replica as the preferred replacement for DFS Replication where DFS has been used as a very low-end DR solution in specific environments, some situations still lend themselves to using DFS. However, both have their use cases.
DFS Replication use cases
DFS is well-suited in a couple of different scenarios, in particular when compared to Storage Replica. One of these is in environments where bandwidth and throughput are minimal. While Storage Replica has an asynchronous configuration that is less bandwidth-intensive, it is really not meant to operate in severely bandwidth-constrained environments. DFS does really well in these low-bandwidth use cases.
DFS is included in legacy versions of Windows Server. If organizations are still leveraging supported legacy Windows Server versions, DFS is the preferred solution. Storage Replica was not introduced until Windows Server 2016, so organizations would need to take this into account when thinking of using Storage Replica.
DFS has many limitations to be aware of when replicating data that can help organizations to make the decision to use Storage Replica instead. These include:
- Inability to replicate files that are in use or open.
- Synchronous replication is not possible with DFS Replication.
- DFS replication can take long periods of time—minutes, hours, or even days.
- The consistency of the DFS database can be finicky, as power outages and other abrupt interruptions can lead to long consistency checks of the DFS DB.
- DFS is a multi-master technology that allows replication to flow in both directions. This can lead to unwanted data overwrites.
Below are the supported configuration limitations for DFS:
- Size of all replicated files on a server: 100 terabytes
- Number of replicated files on a volume: 70 million
- Maximum file size: 250 gigabytes
Storage Replica use cases
Storage Replica is the newer technology that touts many great features and capabilities. Microsoft recommends that organizations use Storage Replica in place of DFS in situations where DFS has been used as a makeshift disaster recovery solution. It is essential to distinguish between a "backup" solution and a "replication" solution. Microsoft does not support Storage Replica as a backup solution.
Several use cases lend themselves to Storage Replica. Storage Replica, unlike DFS Replication, is not a checkpoint-based replication solution. It continuously replicates. With this being the case, the deltas of data are much lower. Storage Replica is not plagued with the open files' constraints found with DFS Replication since it operates below the file level. It makes it a better solution to ensure that all data is replicated, including open files.
There are limitations to be aware of with Storage Replica. These include:
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- There is only a one-to-one relationship between replicated volumes. You can replicate different volumes between servers.
- Even with the asynchronous replication option, Storage Replica is not well-suited for high-latency, low-bandwidth environments.
- Storage Replica data on the destination system cannot be accessed when replication is occurring.
- Truly unlimited Storage Replica capabilities require Windows Server Datacenter edition. With Standard Edition, you are capped at 2 TB of data and 1 volume.
Wrapping up ^
Both DFS Replication and Storage Replica are technologies that allow organizations to replicate data between servers. Both have their strengths and fitting use cases. DFS Replication is still preferred in very low-bandwidth environments and when using legacy Windows versions. Storage Replica is much newer and operates at the volume level, which frees it of many of the open file limitations of DFS. Storage Replica is severely impaired in Windows Server Standard edition. To use the solution to its full potential, organizations need to be running Windows Server Datacenter edition, which is significantly more expensive.