Using RAID

Some notes from aaaaages ago, not necessarily part of our syllabus.

RAID stands for (Redundant Array of Independent Disks) (or originally, Inexpensive disks, unfortunately that’s not the case any more!). Lets break that down into what it really means:

• “Array”, so we’re talking more than one disk drive here.
• “Independent”, these are separate disks that we are linking together via our RAID controller. They could already be on the same bus or data channel – but they are independent devices.
• “Redundant”. We’re using more disks than we need in our array. Why is that? Well, by combining the resources of several hard drives, we can end up with a configuration that is faster, larger, more tolerant or more secure – depending on our needs.

Hardware and Software Implementations

There are a number of different configurations of RAID which have different advantages for different types of application. These configurations are generally known as levels. RAID can usually be set up at hardware or software level. With a hardware system, the hard drive controller must be capable of supporting RAID, and supporting the RAID level you want to use. At when using hardware level the operating system may not even be aware RAID is configured, as it will only see the resultant drive configuration, not the raw disks. Hardware level RAID generally provides better reliability and stability, as well as offering more flexibility when it comes to things like hot swapping hard drives.

Typically to configure your hardware level RAID there will be a configuration tool accessible when you boot the computer, the tool will allow you to group your physical disks and establish RAID configurations. Sometimes a software tool may be provided by your hardware manufacturer, that will allow you to configure changes from within your operating system.

RAID can also be implemented at software level by the operating system, in this scenario the operating system knows to treat the independent disks as an array, and configuration of the array takes place within the operating system (in Windows through the “Disk Management” tool inside “Computer Management”). Software level RAID uses more system resources than hardware, but does not require a special RAID drive controller. It may also not support true hot-swapping.

Raid Levels

You might want to look at this RAID Tutorial for further information

• Level 0
  • This is a very simple configuration where several disks are combined and data is stored across all the disks. Since data is being stored across several disks, a larger volume of data can be processed at once.
  • This configuration is not fault tolerant, which means it is not a “true” RAID configuration – since if any disk in the array fails, the whole array’s data will be lost as file system information will have been lost.
• Level 1
  • This is another simple configuration, in which disks in the array mirror each other. This system, whilst 100% inefficient, provides complete fault tolerance. If one device in the array fails, there is still a mirrored copy.
  • If a disk fails, it can usually be hot-swapped out whilst the computer is still running, making this a good configuration for systems that cannot afford much downtime.
  • RAID 1 and 0 can be combined with RAID 10 in which a RAID 0 configuration is mirrored at RAID 1, to produce a configuration that is both fully fault tolerant and supports a high volume of data. This is great for high power database servers.
• Level 5
  • In a RAID 5 system, data is stored across a number of drives in the array, with disk parity bits distributed across the array. This means that in the event of a disk failing, the missing data could be rebuilt from the parity and remaining data.
  • RAID 5 can obtain high transfer speeds similar to RAID 0 by utilising all the drives in the array.
  • A RAID 5 system requires at least three drives, and is good for more general purpose servers.

Other RAID configurations exist, however these are either more rare, more specialist in their application or more high-end than my needs have ever been.