This document came from in Mar-2003

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RAID Description


Redundant Disk System - RAID
RAID Level Description Performance
Fault Tolerant?
RAID 0 Disk Striping Parallel Disk I/O No
RAID 1 Disk Mirroring None Yes (1 Drive Failure)
RAID 2 Disk Striping with Hamming Code for Error Protection None Yes (1 Drive Failure)
RAID 3 Disk Striping with Dedicated Parity Drive Parallel Disk I/O Yes (1Drive Failure)
RAID 4 Disk Striping with Dedicated Parity Drive; Non- synchronized Disks Required Parallel Disk I/O Yes (1Drive Failure)
RAID 5 Disk Striping with Distributed Parity Parallel Disk I/O (not as Fast as RAID 0) Yes
Redundant Arrays of Independent Disks, or RAID, is a rapidly expanding storage technology which promises a major improvement in the way on-line data is stored in computers.

RAID Level Definitions
Those investing in storage will need to consider low cost per Mbyte, high input/output I/O, and high data reliability in order to obtain a balance to suit their needs.

RAID 0 - Disk Striping
Disk striping writes data across all disks concurrently rather than on one disk at a time. Although termed RAID 0, it is not a true implementation of RAID because there is no facility for redundancy. Therefore, in the event of a disk failure, data is lost.
In the disk array subsystem, data chunk 0 is written to disk 0 , chunk 1 is written to disk 1 and so on. When the last disk is reached and written, the array proceeds to store data on the next level of the first disk.
Disk striping is fast as data can be transferred to multiple disks simultaneously: chunk 0 is still being written to disk 0 while chunk 1 is being written to disk 1. Furthermore, reads and writes can overlap.
An example of a typical usage for RAID 0 could be: Data from the field comes into the central processing location on tape where it is instantly processed. Redundancy is not a requirement as the tape can be relocated.
Summary: RAID 0 offers the highest performance without redundancy. Some industries that RAID 0 is particularly suited to are: meteorology, geophysical exploration, oil and gas industries, video/graphics.

RAID 1 - Disk Mirroring
Disk mirroring protects against disk failure by keeping two copies of data stored on separate disks or arrays. Though simple and easy to implement, installing two sets of disks effectively doubles the investment required for a single, non-redundant drive. If at any time either disk fails, the remaining disk can provide all of the data needed, preventing downtime.
Two copies of the data also ensure that there is no degradation in performance, as accesses are immediately routed to the working disk. In the event of failure, copying from the operational disk to the replacement disk is very fast, which reduces the risk of a second failure.
RAID 1 not only provides protection, it can also improve performance. For example, if multiple requests for the same data are made, demand can be distributed between two disk copies therefore increasing response time for data access.
Summary: RAID 1 is the most secure of any of the RAID levels and is exceptionally fault-tolerant. Examples of industries that would use this level are those who cannot afford downtime: banks, insurance companies, stock markets, airline systems.

RAID 3 - Parallel Data Access
In RAID 3 data is distributed to a striped array and a disk is added to store redundant information. The array consists of three disks for the data and one parity disk for the redundancy. In the event of a disk failure, data can be mathematically reconstructed from the remaining disks in the array.
Synchronization enables striped data to be read and written as quickly as possible. However, when multiple writes are involved, performance is reduced because the parity drive has to be accessed for every single write, which may create a bottleneck at the parity drive. Consideration should also be given to impacts on performance as disk rotation must be synchronized before data can be accessed.
Summary: RAID 3 is ideal for intensive high-speed, long data transfer applications such as: video, CAD/CAM, graphic applications, scientific modelling.

RAID 5 - Independent Access Arrays
In RAID 5, the redundancy offered in RAID 3 by a single parity disk, is distributed across all the disks in the array. Data and relative parity are never stored on the same disk.
One user may be writing a chunk to disk 0 and the corresponding parity to disk 3, another user may be writing to chunk 4 of disk 1 and updating parity on disk 2. There is a clear dividend in terms of performance and the speed of transactions.
During disk writes, RAID 5 cannot produce a write performance comparable to that of straight disk striping because other operations have to be undertaken to make and store parity codes. The I/O performance of the array depends very much on the relative levels of reads and writes requested.
When a stripe is modified, unmodified portions must also be read to re-generate the parity for the entire stripe. Once the parity has been generated, the modified data and parity information must be written to disk. This is commonly know as Read/Modify/Write strategy.
It reflects that, though RAID 5 is superior to RAID 0 because it offers redundancy, it is not able to perform as well as RAID 0 in terms of write performance. Because RAID 5 has distributed parity, two reads and two writes must be performed for every write operation. However, the write penalty can be overcome by the use of write caching which allows write data to be stored in the memory prior to writing to the disk, so freeing the host processor for other tasks.
Summary: RAID 5 is ideal for organizations running databases and other transaction-based applications such as: banks, airline and railway reservation systems, government departments, utilities and telecommunications.

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