NAS mirror backup is a simple and effective technique that synchronizes two NAS appliances and ensures both contain duplicate data. Like any other backup technique, there are both advantages and disadvantages to NAS mirror backups.
The main advantage to creating a NAS mirror backup is that the secondary NAS appliance mirrors the primary device. If the primary network-attached storage appliance was to fail, then the secondary NAS could serve as an immediate replacement without the need for a traditional backup restoration. NAS mirroring can be an effective way for an organization to protect its data against hardware failure.
At the same time, NAS mirroring lacks some of the protections associated with other types of backup. Due to these limitations, organizations will typically need to use NAS mirroring in conjunction with other protective mechanisms, such as snapshots or supplementary point-in-time backups.
Cost and support barriers
One of the biggest disadvantages to a NAS mirror backup is the cost. To perform a NAS mirror backup, an organization needs duplicate NAS appliances that run identical firmware versions and are equipped with similar disks. All this hardware comes at a cost, especially when NAS mirroring typically does not support compression or other data reduction techniques. Depending on how the backup is being performed, admins may require additional software. Some network-attached storage appliances natively support NAS mirroring, but others require the use of third-party software.
If an organization's network-attached storage appliances do not natively support hardware-level NAS mirroring, then there are two main approaches that the organization can use to mirror its NAS devices. The first option is to perform file-level mirroring with a utility, such as AOMEI Backupper or Allway Sync.
The other approach involves using the replication capabilities that are built into a hypervisor. Use a primary NAS that contains several virtual hard disks and has the Hyper-V replication feature to replicate these virtual hard disks to a secondary NAS device. This approach might not count as NAS mirroring in the truest sense of the word, but still results in two NAS appliances with identical contents.
Beware of data protection limits
Another potential disadvantage to NAS mirror backups is that, by itself, NAS mirroring offers limited protection for an organization's data. In fact, there are two inherent data protection limitations.
The first limitation is in the frequency with which backups are created -- the recovery point objective. Some of the NAS appliances that natively enable NAS mirroring at the hardware level use synchronous data replication, writing data to both appliances at nearly the same time. However, when replication is handled at the software level, it usually occurs asynchronously. This means that there is a delay between the time that data is written to the primary NAS appliance and the time that the data is written to the secondary appliance.
Every organization's synchronization frequency will be different depending on the method. Admins must consider the amount of data that could be lost with an asynchronous replication process.
The other limitation to data protection with NAS mirror backups is that any change an organization makes to the data on a primary NAS is replicated to the secondary device. If a user accidentally deletes a file or if the organization suffers file encryption due to a ransomware infection, then those changes will be written to the secondary NAS appliance.
In other words, NAS mirroring can only be thought of as a substitute for a traditional backup if the mirroring method also offers some sort of point-in-time recovery mechanism. Some NAS appliances enable admins to automatically create periodic snapshots that they can use to revert the appliance's contents to a previous state if necessary. Another common fix is to run a traditional backup against the contents of the secondary network-attached storage appliance.