Using Drive Capacity Efficiently - 3Ware 9550SXU-12 User Manual

Using Drive Capacity Efficiently

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Table 4: Possible Configurations Based on Number of Drives
# Drives Possible RAID Configurations
5 RAID 6
RAID 5 with hot spare
RAID 10 with hot spare
Combination of RAID 0, RAID 1, hot spare, single disk
6 or more RAID 6
RAID 6 with hot spare
RAID 50
Combination of RAID 0, 1, 5, 6,10, hot spare, single disk
To make the most efficient use of drive capacity, it is advisable to use drives
of the same capacity in a unit. This is because the capacity of each drive is
limited to the capacity of the smallest drive in the unit.
The total unit capacity is defined as follows:
Table 5: 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 6 (number of drives - 2) x (capacity of the smallest drive)
RAID 10 (number of drives / 2) X (capacity of smallest drive)
RAID 50 (number of drives - number of groups of drives) X (capacity of the
smallest drive)
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
Understanding RAID Concepts and Levels
13