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Raid Levels

RAID stands for Redundant Array of Inexpensive Disks. A RAID system consists of two or more disks working in parallel. They appear as one drive to the user, and offer enhanced performance or security (or both).

The software to perform the RAID-functionality and control the hard disks can either be located on a separate controller card (a hardware RAID controller) or it can simply be a driver. Both Windows NT 4 and 2000 include a software RAID solution. Hardware RAID controllers cost more than pure software but they also offer better performance.

Most RAID-systems are based on SCSI, although implementations using IDE disks or FC (fibre channel) disks also exist. There are even systems that use IDE disks internally but that have a SCSI-interface for the host system.

There are different RAID levels, each suiting specific situations. RAID levels are not standardized by an industry group. This explains why companies are sometimes creative and come up with their own unique implementations.

Sometimes disks in a RAID system are defined as JBOD, which stands for 'just a bunch of disks'. This means that those disks do not use a specific RAID level and are used as if they were stand-alone disks. This is often done for disks that contain swap files or spooling data.

Below is an overview of the most popular levels:

RAID 0: Striping

In a RAID 0 system, data are split up in blocks that get written across all the drives in the array. By using multiple disks (at least 2) at the same time, RAID 0 offers superior I/O performance. This performance can be enhanced further by using multiple controllers, ideally one controller per disk.

Advantages

RAID 0 offers great performance, both in read and write operations. There is no overhead caused by parity controls.
All storage capacity can be used, there is no disk overhead.
The technology is easy to implement.

Disadvantages

RAID 0 is not fault-tolerant. If one disk fails, all data in the RAID 0 array are lost. It should not be used on mission-critical systems.

Ideal Use

RAID 0 is ideal for non-critical storage of data that have to be read/written at a high speed, e.g. on a PhotoShop image retouching station.

RAID 1: Mirroring

Data are stored twice by writing them to both the data disk (or set of data disks) and a mirror disk (or set of disks) . If a disk fails, the controller uses either the data drive or the mirror drive for data recovery and continues operation. You need at least 2 disks for a RAID 1 array.

RAID 1 systems are often combined with RAID 0 to improve performance. Such a system is sometimes referred to by the combined number: a RAID 10 system.

Advantages

RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single disk.
In case a disk fails, data do not have to be rebuild, they just have to be copied to the replacement disk.
RAID 1 is a very simple technology.

Disadvantages

The main disadvantage is that the effective storage capacity is only half of the total disk capacity because all data get written twice.
Software RAID 1 solutions do not always allow a hot swap of a failed disk (meaning it cannot be replaced while the server keeps running). Ideally a hardware controller is used.

Ideal Use

RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two disks will be used.

RAID 3

On RAID 3 systems, datablocks are subdivided (striped) and written in parallel on two or more drives. An additional drive stores parity information. You need at least 3 disks for a RAID 3 array.

Since parity is used, a RAID 3 stripe set can withstand a single disk failure without losing data or access to data.

Advantages

RAID-3 provides high throughput (both read and write) for large data transfers.
Disk failures do not significantly slow down throughput.

Disadvantages

This technology is fairly complex and too resource intensive to be done in software.
Performance is slower for random, small I/O operations.

Ideal use

RAID 3 is not that common in prepress.

RAID 5

RAID 5 is the most common secure RAID level. It is similar to RAID-3 except that data are transferred to disks by independent read and write operations (not in parallel). The data chunks that are written are also larger. Instead of a dedicated parity disk, parity information is spread across all the drives. You need at least 3 disks for a RAID 5 array.

A RAID 5 array can withstand a single disk failure without losing data or access to data. Although RAID 5 can be achieved in software, a hardware controller is recommended. Often extra cache memory is used on these controllers to improve the write performance.

Advantages

Read data transactions are very fast while write data transaction are somewhat slower (due to the parity that has to be calculated).

Disadvantages

Disk failures have an effect on throughput, although this is still acceptable.
Like RAID 3, this is complex technology.

Ideal Use

RAID 5 is a good all-round system that combines efficient storage with excellent security and decent performance. It is ideal for file and application servers.

What About RAID 2, 4, 6, or 7?

These levels do exist but are not that common, at least not in prepress environments. This is just a simple introduction to RAID-systems.

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