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Chapter 1. Introduction to the 3ware Command Line Interface
14 3ware Serial ATA RAID Controller CLI Guide for the Power Mac G5
Using Drive Capacity Efficiently
To make the most efficient use of drive capacity, it is advisable to use drives
of the same capacity. This is because the capacity of each drive is limited to
the capacity of the smallest drive in the unit.
The total array capacity is defined as follows:
Through drive coercion, the capacity used for each drive is rounded down so
that drives from differing manufacturers are more likely to be able to be used
as spares for each other. The capacity used for each drive is rounded down to
the nearest GB for drives under 45 GB (45,000,000,000 bytes), and rounded
down to the nearest 5 GB for drives over 45 GB. For example, a 44.3 GB
drive will be rounded down to 44 GB, and a 123 GB drive will be rounded
down to 120 GB. For more information, see the discussion of drive coercion
under “Creating a Hot Spare” on page 42.
Table 3: Drive Capacity
RAID Level Capacity
Single Disk capacity of the drive
RAID 0 (number of drives) X (capacity of the smallest drive)
RAID 1 capacity of the smallest drive
RAID 5 (number of drives - 1) X (capacity of the smallest drive)
Storage efficiency increases with the number of disks:
storage efficiency = (number of drives -1)/(number of drives)
RAID 10 (number of drives / 2) X (capacity of smallest drive)