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Raid Explained

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RAID Explained
RAID used to stand for "redundant array of inexpensive disks". Today the term has been updated to "redundant array of independent disks". RAID is a way of grouping individual physical drives together to form one bigger drive called a "RAID set". RAID can make many smaller disks appear as one large disk to a server. The RAID set represents all the smaller physical drives as one logical disk to your server. The logical disk is called a LUN, or "logical unit number". Using RAID has two main advantages. Better performance and higher availability, which means it goes faster and breaks down less often.
RAID benefits explained
Performance is increased because the server has more "spindles" to read from or write to when data is accessed from a drive. Availability is increased because the RAID controller can recreate lost data from parity information.
What is parity? Parity is basically a checksum of the data that was written to the disks, which gets written along with the original data. RAID can be done in software on a host, or in hardware on the storage controllers. The server accessing the data on a hardware-based RAID set never knows that one of the drives in the RAID set went bad. The controller recreates the data that was lost when the drive went bad, by using the parity information stored on the surviving disks in the RAID set.
There are a number of different ways drives can be grouped together to form RAID sets. The different methods used to group drives are called "RAID types". RAID types are numbered from 0 to 5. The numbers represent the "level" of RAID being used. RAID levels 0, 1 and 5 are the most common.
The RAID type you should use depends on the type of application you are running on your server. RAID-0 is the fastest. RAID-1 is the most reliable and RAID-5 is a good combination of both.
RAID types explained
Below is a description of the different types of RAID that most commonly used in SAN storage arrays. Not all storage array vendors support all the various RAID types. Check with your vendor for the types of RAID that are available with their storage.
A storage area network (SAN) is a high-speed special-purpose network (or subnetwork) that interconnects different kinds of data storage devices with associated data servers on behalf of a larger network of users.

RAID-0: RAID-0 is called disk "striping". All the data is spread out in chunks across all the disks in the RAID set. RAID-0 has great performance, because you spread out the load of storing data onto more physical drives. There is no parity generated for RAID-0. Therefore there is no overhead to write data to RAID-0 disks. RAID-0 is only good for better performance, and not for high availability, since parity is not generated for RAID-0 disks. RAID-0 requires at least two physical disks. RAID-1: RAID-1 is called disk mirroring. All the data is written to at least two separate physical disks. The disks are essentially mirror images of each other. If one of the disks fails, the other can be used to retrieve data. Disk mirroring is good for very fast read operations. It's slower when writing to the disks, since the data needs to be written twice. RAID-1 requires at least two physical disks. RAID-5: RAID-5 uses disk striping with parity. The data is striped across all the disks in the RAID set, along with the parity information needed to reconstruct the data in case of disk failure. RAID-5 is the most common method used, since it achieves a good balance between performance and availability. RAID-5 requires at least three physical disks.

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