RAID

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What is RAID?

RAID is an acronym for Redundant Array of Inexpensive (or sometimes “Independent”) Disks. The RAID system is a method of combining several hard drives into one logical unit offering fault tolerance and much higher throughput levels than a single drive or group of independent drives.

Why RAID?

	RAID provides real-time data recovery with uninterruptible access when a hard drive fails.
	RAID increases system up-time and network availability.
	RAID protects against data loss.
	RAID increases system performance accessing multiple drives working in parallel.

Levels of RAID

	RAID 0	RAID 1	RAID 5
Description	Data Striping (No Data Protection)	Disk mirroring	Data parity with distributed parity
Min Number of Drives	2	2	3
Benefit	Highest performance	Data protection through redundancy	Best balance of cost / performance / data protection

You can also combine the levels of RAID like this

	RAID 0/1	RAID 0/5
Description	RAID 0 and RAID 1 combined	RAID 0 and RAID 5 combined
Min Number of Drives	4	6
Benefit	Highest performance with data protection	Provides increased fault tolerance

The types of RAID

Software-based RAID

Hardware-based RAID

External Hardware RAID

Description

Included in NOSs such as NetWare and Windows NT®. All RAID functions are handled by the host CPU which can severely tax its ability to perform computations.

Processor-intensive RAID operations are off-loaded from the host CPU to enhance performance.

Connects to the server via a standard controller. RAID functions are performed on a microprocessor located on the external RAID controller independant of the host.

Advantages

	Low Price
	Only requires a standard controller

	Data protection and performance benefits of RAID.
	More robust fault-tolerant features and increased performance versus software-based RAID

	OS independent
	Build super high - capacity storage systems for high-end servers

Server technology comparison

ATA

SCSI

Fibre Channel

Best Suited For

Low-cost entry level server with limited expandability

Low to high-end server when scalability is desired

Server-to-server campus networks

Advantages

Uses low-cost ATA drives

	Performance: up to 160 MBytes/sec
	Reliability
	Connectivity to the largest variety of peripherals
	Expandability

	Dual active loop data path capability
	Infinitely scalable

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