Disk array capacities

These guidelines will help you calculate the capacity of a disk array.

The capacity of a disk array depends on the capacity of the disks used and the RAID level of the array. To calculate the capacity of a disk array, use the following methods:

RAID 0

Multiply the number of disks by the disk capacity.

RAID 5

Multiply one fewer than the number of disks by the disk capacity.

RAID 6

Multiply two fewer than the number of disks by the disk capacity.

RAID 10

Multiply the number of disks by the disk capacity and divide by 2.

RAID 5T2, 6T2, and 10T2

Each tier in the array follows the capacity rules for the base RAID level of the tier. Note that each tier must contain at least 10% of the total disk capacity. The disk capacity per tier is calculated by taking the smallest drive in each tier multiplied by the total number of physical disks in that tier. When you divide the disk capacity of each tier by the total disk capacity, the result must be larger than 10%.

For example, if you are creating a 5T2 Easy Tier array with three 387 GB SSDs and eight 857 GB HDDs, the SSD tier capacity would be 3x387 / ((3x387) + (8x857)) which would result in a SSD tier that is 14.5% of the total disk capacity.

If the resulting percentage of any of the tiers is less than 10%, the array create operation might fail with the following error: Command failed, Mixed Block Device classes: The multiple drive types are incompatible with the RAID level specified and cannot be mixed together.

Note: If disks of different capacities are used in the same array, all disks are treated as if they have the capacity of the smallest disk. For a tiered array, each tier uses the capacity of the smallest disk within that tier.