View source for RAID

===Types of RAID===
====RAID 0====

RAID 0 requires at least one disk.  In this array configuration stripes are written and read across however many disks are in the array.  Typically this leads to an increase which is, assuming there are no subsystem bottlenecks, almost a 100% increase per disk in the array compared to a single disk.  

Unfortunately this array type is more susceptible to failure, as it provides no redundancy and a single disk failure will take down the array.

Single disk RAID 0 arrays are sometimes used by server administrators to quickly get a box up and running before they add more disks later, and migrate the array to another striped array type, such as RAID 5.

There is no capacity loss on this array type to parity, mirroring, or anything else.  The storage capacity of RAID 0 can be expressed as size*n where n is the number of disks.

====RAID 1====

RAID 1 requires two disks.  Effectively, this array configuration writes the same data to both disks concurrently, creating a mirror.  This array configuration can sustain a single disk loss without data loss.  Some controllers will also offer a read throughput increase on this array type.

Some RAID 1 controllers actually implement striped mirror sets or mirrored stripe sets (RAID 01 or 10) in place of RAID 1.  This affords the ability of the array to be expanded later, or even migrated to another array type.  Usually this is transparent to the user, and the controller still refers to it as RAID 1.

Obviously an array of this type has only 50% of the storage space of the combined physical disks.  The storage capacity of RAID 1 can be expressed as size(n/2) where n is the number of disks.

====RAID 5====

RAID 5 arrays generally require a three disk minimum.  RAID 5 uses striping with distributed parity to achieve redundancy and a throughput increase.  On a three disk RAID 5 array, two disks will receive a stripe of data, and the other will receive a stripe of parity.  Which disks receive what is rotated, so that the parity is distributed amongst the disks.  RAID 5 can sustain the loss of a single disk without losing data.  However, unlike RAID 1, RAID 5 has a greater storage capacity for any given number of disks.  

Sometimes RAID 5 arrays are created with two disks, and are effectively the same as a 3 disk RAID 5 array which has lost a single disk.  In this state the array will be degraded in performance as all reads will require parity calculations, as well as having no redundancy.  A RAID 5 array in this state will go down if a disk is lost.  A third disk needs to be added, and the array rebuilt before performance and redundancy return to normal.  Two disk RAID 5 arrays are very rarely created.

The storage capacity of RAID 5 can be expressed as size*(n-1) where size is the size of the smallest disk and n is number of disks.

====RAID 6====

RAID 6 is similar to RAID 5 with double the parity.  That is, it uses striping with double distributed parity.  Opposed to every two chunks from a stripe being used to calculate one chunk of parity, two chunks of parity are created, and then distributed as with RAID 5.  This method effectively ''doubles'' the redundancy of the array: a RAID 6 array with a four-disk minimum will fail after two disks have been lost.  Using a four disk RAID 6 array as an example, two disks will effectively be used for striping the actual data, the remaining two are used for storing the parity information.  Using this configuration, any fears of a second disk failing or of a write error occurring whilst replacing a failed disk are mitigated.  

The storage capacity of a RAID 6 array can be expressed as size*(n-2) where size is the size of the smallest disk and n is the number of disks.

====Nested RAID Levels====

RAID 5 and RAID 6 arrays can give particularly good value and performance in regards to the effective use of disks.  However, as the number of disks in the array increases, the redundancy of a RAID 5 or RAID 6 array can decrease to and below that of a single disk.  That is, regardless of how many disks are in the array, a RAID 5 array will go down once a second disk has failed, likewise, a RAID 6 array '''will''' go down once a third disk has failed.  

To overcome this as the size of the array increases, RAID 1 is an obvious choice, as it's redundancy scales with the size of the array, however, it doesn't come with the performance benefits you might expect from striped arrays such as RAID 5 and RAID 6.  This is where we introduce a group of RAID levels known as "Nested".  These RAID levels can have better scalability with regards to redundancy and occasionally performance.  These RAID levels work by combing two or more RAID levels and applying it to the entire array.

=====RAID 10=====

RAID 10, also known as RAID 1+0, works by splitting the entire group of disks into two or more groups, striping over all the groups, and then creating a local mirror in each group.  This is where the name becomes evident, as it combines RAID 1 with RAID 0.  For example, if I had a four-disk RAID 10 array, you can imagine a single piece of data being split into two stripes, one stripe going to Group A, the other going to Group B.  Group A then mirrors it's stripe across it's two disks, Group B mirrors it's stripe across it's two disks in a similar fashion.  So we are effectively creating two RAID 1 arrays within a larger RAID 0 array.

A four-disk RAID 10 array will sustain a '''maximum''' of two disk failures before the array goes down.  That is, it's dependent on which two disks fail.  Imagine, for example, that both disks in the Group A RAID 1 of the example above failed, then we have lost an entire group out of the higher RAID 0 array:  the entire array will fail.  However, if one disk from Group A failed, and one disk from Group B failed, the array will continue to function, as both of the RAID 1 arrays are still active.  Note that if a third disk were to fail, the array will go down regardless of which RAID 1 batch it was in, as with a four-disk array it would indicate an entire RAID 1 array had been lost.

Depending on the RAID controller, RAID 10 arrays are typically limited to four disks.

The storage capacity of a RAID 10 array is expressed in the same fashion as a RAID 1 array: size*(n/2) where size is the size of the smallest disk and n is the number of disks.