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How do you calculate the space available when you configure three disks of different capacity to RAI

I have three disks, each different sizes (60 GB, 80 GB and 40 GB) and I have configured them to RAID 5. How do you calculate the disk space available for data storage and how much disk space is utilised for parity storage?

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A RAID set can only utilise space that is available on every drive, meaning that in the example above each drive is calculated at 40GB for RAID purposes. The remaining capacity on the 60GB and 80GB drive is wasted and cannot be used for RAID. This leaves you with 120GB of RAW disk space on the RAID set.

Now onto the RAID calculation – The RAID 5 standard is an 'n+1' RAID level, with 'n' data drives and one parity drive per RAID set. The fact that in RAID 5 the parity is striped across all drives is immaterial. In our configuration there will be two data drives and one parity drive, allowing us 80GB of usable disk space for our 120GB raw disk space.

As can be seen, the usable capacity of the original three drives has been reduced from 180GB to only 80GB, which is around 45% utilisation and not ideal or recommended. If these are the only drives available you have achieved your goal of RAID protection, although at a high cost in terms of wasted capacity.

If you are looking for best practices you should always try to use disks of the same size in a RAID set to avoid unnecessary wastage. Also, if you have one RAID set of 40GB drives and another RAID set of 80GB drives you would need a hot spare drive of 80GB in size. This is because a larger hot spare can replace a failed disk of smaller capacity but a smaller hot spare cannot replace a failed disk of larger capacity.

This was first published in June 2009

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