Volume Groups’ usage for backup targets (Windows Server)

  • 31 July 2020
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As you already know Nutanix provides block storage services in your clusters which are called Volume Groups. This provides SAN like capabilities by leveraging the existing hyper-converged infrastructure. Additionally, volume groups support SCSI unmap commands natively without having to turn on any additional setting as of AOS 4.7 in order to reclaim space from deleted blocks, more on that in the Nutanix Bible in the “Volumes (Block services)” section. 

 

However, there are certain caveats when using volume groups as backup targets so you must architect your backup solution wisely as you may have to consider losing some benefits in order to gain others.

In this example, I will focus on Veeam’s feature of Fast clone, which you can read more about Veeam’s documentation here. However, this could apply to any backup solution that leverages underlying features from the file system to provide storage savings. 

 

In essence, this solution leverages the block cloning feature from ReFS in Windows which it allows referencing existing blocks in the storage device, for each physical region of data written on the disk, there is a reference count, which means if a new write operation comes in that matches an already written block, the file system will update the reference count for that block adding an extra entry for this write, transforming this data operation into a metadata update, this translates into storage savings for the storage device, which is really useful considering how much of the data from backups jobs can be deduplicated whether they are full or incremental.

However, as of right now ReFS doesn’t support scsi UNMAP for iscsi LUNs, it only supports it for Windows storage spaces and Directly attached devices, and even for those options it comes disabled by default, more on that here

This means until this limitation is removed, Microsoft’s recommendation for backup targets is to use NTFS when formatting your LUNs if scsi UNMAP is required as shown below.

 

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Reference: ReFS overview

 

In conclusion, depending on your environment you will need to decide which feature will be more valuable, either get the storage savings from block cloning and provision a new LUN as a backup target once it gets full or having the ability to reclaim the space from the LUN once that gets deleted.


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