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Determining What RAID Level to Use
Your choice of which type of RAID unit (array) to create will depend on your needs. You may wish to maximize speed of access, total amount of storage, or redundant protection of data. Each type of RAID unit offers a different blend of these characteristics.
The following table provides a brief summary of RAID type characteristics.

RAID Configuration Types
RAID Type | Description
RAID 0 | Provides performance, but no fault tolerance.
RAID 1 | Provides fault tolerance and a read speed advantage over non-RAID disks.
RAID 5 | This type of unit provides performance, fault tolerance, and high storage efficiency. RAID 5 units can tolerate one drive failing before losing data.
RAID 6 | Provides very high fault tolerance with the ability to protect against two consecutive drive failures. Performance and efficiency increase with higher numbers of drives.
RAID 10 | A combination of striped and mirrored units for fault tolerance and high performance.
RAID 50 | A combination of RAID 5 and RAID 0. It provides high fault tolerance and performance.
Single Disk | Not a RAID type, but supported as a configuration. Provides for maximum disk capacity with no redundancy.

You can create one or more units, depending on the number of drives you have installed.

Possible Configurations Based on Number of Drives
# Drives | Possible RAID Configurations
1 | Single disk or hot spare
2 | RAID 0 or RAID 1
3 | RAID 0 RAID 1 with hot spare RAID 5
4 | RAID 5 with hot spare RAID 10 Combination of RAID 0, RAID 1, single disk
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

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:

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 down to 120 GB. For more information, see the discussion of drive coercion under Creating a Hot Spare.
Support for Over 2 Terabytes
Windows 2000, Windows XP (32-bit), 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.

Related Topics
  RAID Concepts
  Available RAID Configurations
  Getting Started with Your 3ware RAID Controller
  Configuring Units

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