Computer RAID

RAID

 RAID is an acronym for "Redundant Array of Inexpensive Disks".

 Configuration for multiple hard drives which provide fault tolerance and improved data access times.

 RAID was traditionally only found in the domain of servers

 But inexpensive IDE RAID solutions now mean many desktop computers can benefit from the same data redundancy, and performance increases for applications like video editing.

Implement RAID

 RAID is a technology that uses multiple hard drives to increase the speed of data transfer to and from hard disk storage.


 Also to provide instant data backup and fault tolerance for any information you might store on a hard drive.

 The hard drives are joined in an array (a single logical drive, as far as the operating system is concerned) and all disks share the data written to them in some form.

 There are several different implementations, or 'levels' of RAID, ranging from RAID 0 to RAID 53.

RAID array:

 A group of hard drives linked together as a single logical drive.

 Must be connected to one or more hardware RAID controllers

 or be attached normally to a computer using a RAID capable operating system, such as Windows XP Professional.
Striping:

 A procedure in which data sent to a RAID array is broken down and portions of it written to each drive in the array.

 This can dramatically speed up hard drive access when the data is read back, since each drive can transfer part of the data simultaneously.

 Striping data on two or more drives actually reduces reliability

 If a single drive in the array fails, all data is lost as each physical hard disk only contains a fragment of the data which is useless without the rest.
Mirroring:

 A procedure in which data sent to a RAID array is duplicated and written onto two or more drives at once.


Parity & Common Types of RAID

 In the majority of RAID implementations, a whole drive, or an area of one or more of the drives in the array is dedicated to storing parity information.

 Each time a bit of information (a digital 1 or 0) is written to every drive in a striped RAID array, an additional parity bit is generated and stored.

 If one of the data drives fails, a new drive can be added and by comparing the information present on the surviving data drive with the corresponding parity information from the parity drive,

 The missing information can be written onto the replacement drive a bit at a time.

 RAID technology began as a method to provide additional data security to business servers

 Many of the RAID levels are still almost exclusively used in the business domain, due to the cost of the required hardware.

 Since the lower levels of RAID are easily implemented on modern computers and need only a pair of drives and a RAID-capable drive controller (hardware) or operating system (software)

 RAID 0 and RAID 1 implementations have become common in the high end desktop/PC

 RAID 0 is used to gain additional performance from conventional drives by pairing them up

 While RAID 1 provides a very simple and effective form of backup by duplicating or 'mirroring' all data on a second drive.

Types of RAID

 Most Hardware RAID controllers intended for the enthusiast or small business markets support only three levels of RAID; RAID 0, 1 and 0+1.

 These are the only levels of RAID that do not require the use of parity, as this feature adds greatly to the complexity and expense of the controller.

RAID 0

• RAID 0 uses multiple hard drives to stripe data over one large logical drive.

• While there are physically two drives, the computer logically sees just one.

• The RAID 0 configuration is typically used when there are data-intensive applications because it offers the fastest data access, though no redundancy




 RAID 0 can essentially combine two hard drives into one using striping, and greatly increase the speed that the drives transfer data.

 This has one obvious disadvantage. There is no fault tolerance.

 If any drive fails, all the data is lost.

RAID 1

• Fault tolerance is the cornerstone of RAID 1.

• In this configuration, two identical physical drives are used, with one drive mirroring the information on the other.

• A RAID 1 configuration is ideal for data redundancy, though storage is more costly as only 1/2 the total drive space of both hard drives is available.



 A mirrored disk array is composed of a set of two physical hard drives, each of which contains a full copy of all data sent to the logical drive that represents the array.

 This has a couple of advantages :

1. Any data stored on a RAID 1 array is completely and automatically backed up, and in the event of the failure of one drive, the other can be substituted without a hitch.

2. Secondly, data can be read from both drives simultaneously, increasing the speed of data retrieval.

 In the event one of the drives in the array fails, a new drive can be added, the array rebuilt, and the RAID controller will duplicate the information onto the new blank drive.

 The disadvantage of RAID 1 is that unlike striping, a mirrored array can use only half of its total free space for storage, since one disk is an exact duplicate of the other.

RAID 1+0

 This RAID level combines the best features of RAID 0 and 1. (Striped array with mirroring)

 It requires a minimum of four physical drives to implement, so it is not cheap.

 Essentially, two pairs of striped drives are mirrored together to provide fault tolerance.

 The mirroring provides the fault tolerance, though if any drive is lost, it must be immediately replaced and the array rebuilt, since it cannot handle the loss of more than one drive.

 Intended for business use, these levels of RAID use the parity system as explained above to provide varying levels of fault tolerance.

 RAID solutions at this level generally come as an add-in controller card or a dedicated storage rack and are intended to work hand-in-hand with hot-swappable hard drive mountings.

 With this setup, any failed drives can be swapped out for new ones on the fly, and the missing data quickly restored by using the parity data.

Hardware & Software RAID

 Depends on your means and expectations.

 Windows XP Pro at least, much easier to set up and much more flexible in terms of disk use than a hardware based system.

 A second factor to consider is whether you want your operating system disk to be part of the RAID array you create?

 The software solution provided by Windows 2000 or XP as it is easier and cheaper.

Using RAID :

 To store a high capacity of data

 Suitable for server

 A system back up

 Many level of RAID from RAID 0 to RAID 53