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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