RAID information and definitions

Begin with the basics

What is RAID?

RAID is an acronym that stands for redundant array of inexpensive disks. RAID is a method of combining several hard drives into one unit. It can offer fault tolerance and a higher throughput levels than a single hard drive or group of independant hard drives.

Why do we need it?

RAID provides a real-time data recovery when a hard drive fails, increasing system uptime and network availability while protecting against loos of data. Multiple drives working together also increase system performance.
Raid Level Description Min No of Drives Benefit
RAID 0 Data striping 2 Highest performance
RAID 1 Disk mirroring 2 High data protection
RAID 0/1 RAID 0 and RAID 1 combined 4 Highest performance with highest data protection
RAID 5 Data striping with distributed parity 3 Best cost/performance balance for multi-drive envoronments.

Compare the types of RAID implementations

Type Description Advantages
Software-based RAID Included in network operating systems such as netwrae and windows nt. All RAID functions are handled by the host CPU which can severly tax its ability to perform other computations
  • Low price
  • Only requires a standard SCSI card
Hardware-based SCSI RAID card Processor-intensive RAID operations are offloaded from the host CPU to enhance performance.
  • Data protection and performance benefits of RAID
  • Connectivity benefits of standard SCSI card
  • More robust fault-tolerant features and increased performance versus software-based RAID
External hardware RAID card Connects to the server via a standard SCSI card. RAID functions are performed on a microprocessor located in the external RAID storage subsystem.
  • Operating system independant
  • Works with any operating system
  • Build super high-capacity storage systems for high end servers

RAID Glossary

Array
Two or more hard disk drives grouped together to appear as a single device to the host computer.
Cache
A temporary, fast storage area that holds data from a slower storage device for quick access. Cache storage is normally transparent to the accessing device.
Co-processor
A processor that resides on an array that relieves the host CPU from executing processor intensive operations such as RAID 5 parity calculations and secondary RAID 1 writes.
Duplexing
Mirroring across two RAID cards.
Fault Tolerance
The ability of a system to continue to perform its functions even when one or more hard disk drives have failed.
Hot Spare
A spare hard drive which will automatically be used to replace the failed member of a redundant disk array.
Hot Swap
The ability to remove a failed member of a redundant disk array and replace it with a good drive without bringing down the server or interupting transactions that involve other devices.
Mirroring(RAID 1)
Provides data protection by duplicating all data from a primary device on a second drive.
NOS
Network operating system (i.e., Netware, Windows NT server, Linux)
Parity
A form of data protection used by RAID level 5 to recreate the data of a failed drive in a disk array.
RAID Levels
Numbered 0 through 5, RAID levels refer to different array architectures that offer various advantages in terms of data availability, cost and performance. RAID levels 0, 1, 0/1, and 5 are the most popular.
RAID 0
See "Striping"
RAID 0/1
Combines RAID 0 (data striping) and RAID 1 (disk mirroring).
RAID 1
See "Mirroring"
RAID 5
Combines data striping (for enhanced performance) with distributed parity (for data protection) to provide a recovery path in the case of failure.
SCSI
A acronym that stands for Small Computer Systems Interface. SCSI is the technology that allows you to connect various devices to a computer. This connection is made using a SCSI card that fits inside your computer.
Striping (RAID 0)
Spreads data evenly over multiple drives to enhance performance. Because there is no redundancy scheme, it does not provide data protection.

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