Nested RAID Levels

Many storage controllers allow RAID levels to be nested. That is, one RAID can use another as its basic element, instead of using physical disks. It is instructive to think of these arrays as layered on top of each other, with physical disks at the bottom.
Nested RAIDs are usually signified by joining the numbers indicating the RAID levels into a single number, sometimes with a '+' in between. For example, RAID 10 (or RAID 1+0) conceptually consists of multiple level 1 arrays stored on physical disks with a level 0 array on top, striped over the level 1 arrays. In the case of RAID 0+1, it is most often called RAID 0+1 as opposed to RAID 01 to avoid confusion with RAID 1. However, when the top array is a RAID 0 (such as in RAID 10 and RAID 50), most vendors choose to omit the '+', though RAID 5+0 is more informative.
When nesting RAID levels, a RAID type that provides redundancy is typically combined with RAID 0 to boost performance. With these configurations it is preferable to have RAID 0 on top and the redundant array at the bottom, because fewer disks then need to be regenerated when a disk fails. (Thus, RAID 10 is preferable to RAID 0+1 but the administrative advantages of "splitting the mirror" of RAID 1 would be lost)

RAID 0+1

A RAID 0+1 (also called RAID 01, though it shouldn't be confused with RAID 10) is a RAID used for both replicating and sharing data among disks. The difference between RAID 0+1 and RAID 1+0 is the location of each RAID system — it is a mirror of stripes. Consider an example of RAID 0+1: six 120 GB drives need to be set up on a RAID 0+1. Below is an example where two 360 GB level 0 arrays are mirrored, creating 360 GB of total storage space:
The maximum storage space here is 360 GB, spread across two arrays. The advantage is that when a hard drive fails in one of the level 0 arrays, the missing data can be transferred from the other array. However, adding an extra hard drive to one stripe requires you to add an additional hard drive to the other stripes to balance out storage among the arrays.
It is not as robust as RAID 10 and cannot tolerate two simultaneous disk failures, if not from the same stripe. That is, once a single disk fails, each of the mechanisms in the other stripe is single point of failure. Also, once the single failed mechanism is replaced, in order to rebuild its data all the disks in the array must participate in the rebuild.
To add to the confusion, some controllers that run in RAID 0+1 mode combine the striping and mirroring into a single operation. The layout of the blocks for RAID 0+1 and RAID 10 are identical except that the disks are in a different order. To the smart controller this does not matter and they gain all the benefits of RAID 10 but are still labelled as only supporting RAID 0+1 in their documentation.

RAID 10

A RAID 10, sometimes called RAID 1+0, or RAID 1&0, is similar to a RAID 0+1 with exception that the RAID levels used are reversed—RAID 10 is a stripe of mirrors. All but one drive from each RAID 1 set could fail without damaging the data. However, if the failed drive is not replaced, the single working hard drive in the set then becomes a single point of failure for the entire array. If that single hard drive then fails, all data stored in the entire array is lost.
Extra 120 GB hard drives could be added to any one of the level 1 arrays to provide extra redundancy. Unlike RAID 0+1, all the "sub-arrays" do not have to be upgraded simultaneously.
RAID 10 is often the primary choice for high-load databases, because the lack of parity to calculate gives it faster write speeds.
RAID 10 Capacity: (Size of Smallest Drive) * (Even Number of Drives ) / 2

RAID 30

RAID level 30 is also known as striping of dedicated parity arrays. It is a combination of RAID level 3 and RAID level 0. RAID 30 provides high data transfer rates, combined with high data reliability. RAID 30 is best implemented on two RAID 3 disk arrays with data striped across both disk arrays. RAID 30 breaks up data into smaller blocks, and then stripes the blocks of data to each RAID 3 raid set. RAID 3 breaks up data into smaller blocks, calculates parity by performing an Exclusive OR on the blocks, and then writes the blocks to all but one drive in the array. The parity bit is created using the Exclusive OR is then written to the last drive in each RAID 3 array. The size of each block is determined by the stripe size parameter, which is set when the RAID is created.

Advantages
RAID 30 can sustain one to four drive failures while maintaining data integrity if each failed disk is in a different RAID 3 array.
Offers highest level of redundancy and performance

Disadvantages
Very costly to implement

RAID 100 (RAID 10+0)

A RAID 100, sometimes also called RAID 10+0, is a stripe of RAID 10s. RAID 100 is an example of plaid RAID, a RAID in which striped RAIDs are themselves striped together.
All but one drive from each RAID 1 set could fail without loss of data. However, the remaining disk from the RAID 1 becomes a single point of failure for the already degraded array. Often the top level stripe is done in software. Some vendors call the top level stripe a MetaLun, or a Soft Stripe.
The major benefits of RAID 100 (and plaid RAID in general) over single-level RAID are better random read performance and the mitigation of hotspot risk on the array. For these reasons, RAID 100 is often the best choice for very large databases, where the underlying array software limits the amount of physical disks allowed in each standard array. Implementing nested RAID levels allows virtually limitless spindle counts in a single logical volume.

RAID 50 (RAID 5+0)

A RAID 50 combines the straight block-level striping of RAID 0 with the distributed parity of RAID 5. This is a RAID 0 array striped across RAID 5 elements.
One drive from each of the RAID 5 sets could fail without loss of data. However, if the failed drive is not replaced, the remaining drives in that set then become a single point of failure for the entire array. If one of those drives fails, all data stored in the entire array is lost. The time spent in recovery (detecting and responding to a drive failure, and the rebuild process to the newly inserted drive) represents a period of vulnerability to the RAID set.
The configuration of the RAID sets will impact the overall fault tolerance. A construction of three seven-drive RAID 5 sets has higher capacity and storage efficiency, but can only tolerate three maximum potential drive failures. Because the reliability of the system depends on quick replacement of the bad drive so the array can rebuild, it is common to construct three six-drive RAID5 sets each with a hot spare that can immediately start rebuilding the array on failure. This does not address the issue that the array is put under maximum strain reading every bit to rebuild the array precisely at the time when it is most vulnerable. A construction of seven three-drive RAID 5 sets can handle as many as seven drive failures but has lower capacity and storage efficiency.
RAID 50 improves upon the performance of RAID 5 particularly during writes, and provides better fault tolerance than a single RAID level does. This level is recommended for applications that require high fault tolerance, capacity and random positioning performance.
As the number of drives in a RAID set increases, and the capacity of the drives increase, this impacts the fault-recovery time correspondingly as the interval for rebuilding the RAID set increases.

Jagath Krishnakumar