Why is there only 41 TB of usable hard drive space on my 45 TB array?
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When purchasing storage, people often anticipate that they will be able to use the entire capacity of the drive or storage system. Unfortunately, this is not always the case. You will usually find there is some missing hard drive space and there are a number of reasons for this.
Overhead from formatting the drive reduces its capacity by approximately 10%, which means that a 300 GB drive will have a usable capacity of approximately 270 GB.
Hard drive manufacturers do not calculate capacity in the same way as many operating systems, which further leads to the appearance of missing hard drive space. A hard drive calculates 1 GB as 1,000 MB, while operating systems calculate it as 1,024 MB.
RAID protection, meanwhile, adds capacity overhead. The most common forms of RAID used in the enterprise today are RAID 10, RAID 5 and RAID 6. Each of these imposes a penalty on usable space, depending on their use of mirroring or parity as a means of data protection and carrying out a RAID rebuild. The capacity penalties are approximately 50% for RAID 10, 25% for RAID 6 (dual parity) and 12.5% for RAID 5 (single parity).
In addition, modern disk arrays usually have a number of hot spares, which take the place of production drives in the event that one fails. These also reduce the capacity of the array because they are usually part of the installed capacity.
This was first published in December 2011