Determining What RAID Level to Use

Select the RAID configuration to use based on the applications to be used on the system, whether performance or data protection is of primary importance, and the number of disk drives available for use.

Review the information under Understanding RAID Concepts and Levels to determine the type of RAID configuration most appropriate for your needs and use the tables below to determine what RAID levels are available, based on your particular controller model and the number of available drives.

The RAID configurations available to you are determined by the number of ports on your controller, and the number of drives attached to those ports. You can configure all drives in one unit, or you can configure multiple units, if you have enough drives.

Possible Configurations Based on # of Drives

# Drives	Possible RAID Configurations
1	Single drive or hot spare
2	RAID 0 or RAID 1
3	RAID 0 RAID 1 with hot spare RAID 5
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, for single disk
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 or 4 subunits of 4 drives. Combination of RAID 0, 1, 5, 10, hot spare, and single disk

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:

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)

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. For more information, see the discussion of drive coercion under Creating a Hot Spare.

Support for Over 2 Terabytes

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. The carve size is adjustable from 1024 MB to 2048 MB (default) prior to unit creation.

If a unit over 2 TB was created prior to enabling the auto-carve option, its capacity visible to the operating system will still be 2TB; no additional capacity will be registered. To change this, the unit has to be recreated.

For more information, see Using Auto-Carving for Multi LUN Support.

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Related Topics

Understanding RAID Concepts and Levels

Available RAID Configurations

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