Example: Usable and unusable capacity

Physical drive capacities influence the way you create arrays and logical drives. Drives in an array can be of different capacities (1 GB, or 2 GB, for example), but RAID controllers treat them as if they all have the capacity of the smallest physical drive.

For example, if you group two 2 GB drives and one 3 GB drive into an array, the usable capacity of the array is 2 GB times 3, or 6 GB, not the 7 GB physically available. The 7 GB is the total disk capacity. In the following diagram, usable capacity is labeled as 1 and unusable capacity is labeled as 2.

  2 GB 2 GB 3 GB

Similarly, if you group three 2 GB drives and 1 GB drive into an array, the usable capacity of that array is 4 GB, not the 7 GB physically available. The 7 GB is the total disk capacity. The remaining capacity left on the three 2 GB drive is unusable capacity.

The optimal way to create arrays is to use physical drives that have the same capacity. Doing so avoids unusable capacity

  For the ServeRAID-8 series (SAS) controllers and ServeRAID-7t controller, usable capacity is the same as the total disk capacity. That is, you can use the remaining space to define another logical drive. The segments that make up the logical drive must be the same size on each physical drive. For example, you can group two 1 GB segments with the remaining space from the previous example to define a 3 GB logical drive. In the following diagram, the new logical drive is labeled 2; 4 GB of usable capacity remain, 2 GB on each drive, labeled 3.

  2 GB 2 GB 3GB 3 GB 3 GB

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