AMCC 9000 User Manual

Determining What RAID Level to Use

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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.

Windows 2000, Windows XP, Linux 2.4, and FreeBSD 4.x, do not 
currently recognize unit capacity in excess of 2 TB. 

If the combined capacity of the drives to be connected to a unit 
exceeds 2 Terabytes (TB), you can enable auto-carving when you 
configure your units. 

Auto-carving divides the available unit capacity into multiple 
chunks of 2 TB or smaller that can be addressed by the operating 
systems as separate volumes.

For more information, see “Enabling Auto-Carving for Units 
Larger Than 2 TB” on page 47, and in 3ware 9000 Series Serial 
ATA RAID Controller User Guide, see “Multi LUN Support and 
Auto-Carving.”

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 subunits) X (capacity of 
the smallest drive)