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RAID stands for Redundant Array of Inexpensive Disks.  RAID levels that exists are levels 0 – 7, but the most popular are 0,1,3 and 5.  I will give a brief overview of those levels below.

RAID provides you with redundancy and fault tolerance of your data, in most configurations.  This allows for high availability of your data even when a hard drive completely fails.

Always have hotspares configured.  A hotspare is a disk that is configured to take the place of a failed disk.  When a disk failure in the RAID occurs, the hotspare will add itself to the array to take the place of the failed disk.  The RAID will then begin to rebuild itself, at the same time still providing you access to your data, naturally with a performance hit until the RAID has completed rebuilding itself.

Use hardware RAID whenever possible.  This means using a controller, just like a SCSI controller, to configure and host the RAID.

• Higher performance because the hardware handles the array.
• You can boot from a RAID.  In order to house your operating system on a RAID 1, you must use hardware RAID controllers.
• Greater features and flexibility.
• You can move the controller and hard disks to a different machine and your RAID is still intact.

Avoid software RAID configurations if possilbe.

• You take a major performance hit.
• You cannot boot to a software RAID because the operating system must be running in order to access the RAID.
• You cannot configure hotspares in a software RAID.
• You cannot move the RAID to a different machine.  Only the operating system that configured the RAID has the information required to access and configure the array.
• If your OS crashes, so does your RAID.  All gone, bye bye data.

RAID 0

• No redundancy in this configuration, but allows for the best performance because of it.  Fastest write performance of any RAID configuration.
• At least 2 hard disks are required, usually of the same size and speed.
• Size is equal to the smallest hard drive in the RAID * the number of drives.

RAID 1

• Also referred to as “mirroring”.  Redundancy is is acheived by writing identical data to both drives.  If one drive fails, the other drives has all your data.
• Requires 2 and only 2 hard disks.
• The size of the array is the size of the smallest drive in the array.
• Commonly used for operating systems.

RAID 3

• In an R3 configuration, data gets striped across multiple disks at the byte level.  A single disk is used to store parity information and the failure of any one disk, including the parity disk, will not cause failure of the array.  The major bottleneck in this solution is the single parity disk, which much be accessed every time information is written to the array.
• Requires at least 3 hard disks.  2 for storing data and 1 parity drive.
• Size of the array is equal to the size of the smallest drive * (total number of drives – 1)
• RAID 4 – Same as RAID 3, but the data is stiped in block and not bytes.  This improves the performance of reading data.

RAID 5

• This is the most popular RAID configuration for storing data, such as your database files.  Unlike RAID 3, parity information is striped across all disks in the array, just like the rest of the data.  This gives R5 a performance increase over R3 when writing data.  Like RAID 4, it uses block level striping for improved read performance.  Fault tolerance is maintained by making sure that the parity information for any particular block of data is not written on the same drive that contains the actual data.
• Perfect for storing of data where the majorty of access to the RAID will be read access, especially when configured with smaller stripe sizes.
• Requires at least 3 hard disks.
• Size of the array is equal to the size of the smallest drive * (total number of drives – 1)
• RAID 6 – same as RAID 5, but parity information is written twice.  The difference in R5 and R6 is improved fault tolerance, but with a performance hit.