Understanding RAID Concepts and Levels
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15
Drive Capacity Considerations
The capacity of each drive is limited to the capacity of the smallest drive in
the array. 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), and rounded down
to the nearest 5 GBytes for drives over 45 GB. For example, a 44.3 GB drive
will be rounded down to 44 GBytes, and a 123 GB drives will be rounded.
down to 120 GBytes.
4
RAID 5 + hot spare
RAID 10
Combination of RAID 0, RAID 1, single disk
5
RAID 5 + hot spare
RAID 10 + hot spare
Combination of RAID 0, RAID 1, hot spare
6 or more
RAID 50
Depending on the number of drives, a RAID 50 may contain from
2 to 4 subunits. For example, with 12 drives, possible RAID 50
configurations include 2 subunits of 6, 3 subunits of 4, or 4
subunits of 3. With 10 drives, a RAID 50 will contain 2 subunits of
5 drives each. With 16 drives, a RAID 50 will contain 2 subunits
of 8 drives each or 4 subunits of 4 drives each.
Combination of RAID 0, 1, 5, 10, hot spare, and single disk
Table 3: Drive Capacity
RAID Level
Capacity
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)
RAID 50
(number of drives - number of groups of drives) X (capacity of the
smallest drive)
Table 2: Possible Configurations Based on Number of Drives
# Drives
Possible RAID Configurations
