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.
||Min No of Drives
||High data protection
||RAID 0 and RAID 1 combined
||Highest performance with highest data protection
||Data striping with distributed parity
||Best cost/performance balance for multi-drive
Compare the types of RAID implementations
||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
- Operating system independant
- Works with any operating system
- Build super high-capacity storage systems for high end servers
- Two or more hard disk drives grouped together to appear as a single
device to the host computer.
- A temporary, fast storage area that holds data from a slower storage
device for quick access. Cache storage is normally transparent to the
- 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.
- 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.
- Network operating system (i.e., Netware, Windows NT server, Linux)
- 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
- 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.