XRAID2 calculator

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aks-2 wrote:

We previously had access to an XRAID calculator: http://rdconfigurator.netgear.com/raid/index.html

 

I found this knowledge base article, which describes the principles of RAID, but not how varied disk capacities contribute to the final XRAID capacity.

If you don't want to waste disk space, then with single redundancy you need to make sure the two largest disks are always the same size.  For dual redundancy, you need to make sure the four largest disks are the same size.

If you follow these rules and start from a factory default, the rules for XRAID are easy to state: 

• Single redundancy - "sum the disks and subtract the largest"
• Dual redundancy - "sum the disks and subtract the largest two"

The old X-RAID calculator never handled expansion, and often that created confusion when the volume size was smaller than expected after expansion.. 

If you want to avoid wasting space when expanding, then it is alway safe to

• Add new disks that are at least as large as the largest already in the NAS
• Single redundancy: make sure the largest two disks are the same size
• Dual redundancy: make sure the largest four disks are the same size

If you know the details of your system's RAID groups, there are other sometimes other options - but it is best to address them case by case.  The formulas would be unwieldy, and they would depend on the expansion history of your volume.