What Is RAID?

RAID (redundant array of independent disks, originally redundant array of inexpensive disks) is a storage technology that combines multiple disk drive components into a logical unit. Data is distributed across the drives in one of several ways called "RAID levels", depending on what level of redundancy and performance (via parallel communication) is required. 

RAID is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple physical drives. The physical drives are said to be in a RAID array,[5] which is accessed by the operating system as one single drive. The different schemes or architectures are named by the word RAID followed by a number (e.g., RAID 0, RAID 1). Each scheme provides a different balance between three key goals: resiliency, performance, and capacity.

Following is a brief textual summary of the most commonly used RAID levels.

RAID 0 (block-level striping without parity or mirroring) has no (or zero) redundancy. It provides improved performance and additional storage but no fault tolerance. Hence simple stripe sets are normally referred to as RAID 0. Any drive failure destroys the array, and the likelihood of failure increases with more drives in the array (at a minimum, catastrophic data loss is almost twice as likely compared to single drives without RAID). A single drive failure destroys the entire array because when data is written to a RAID 0 volume, the data is broken into fragments called blocks. The number of blocks is dictated by the stripe size, which is a configuration parameter of the array. The blocks are written to their respective drives simultaneously on the same sector. This allows smaller sections of the entire chunk of data to be read off each drive in parallel, increasing bandwidth. RAID 0 does not implement error checking, so any error is uncorrectable. More drives in the array means higher bandwidth, but greater risk of data loss.

Pros: Better performance – data replicated across drives, no storage overhead as drives are utilized 100%
Cons: Possibility of losing entire data on failure of a single disk
Minimum Disks Required – 2  

 

In RAID 1 (mirroring without parity or striping), data is written identically to two drives, thereby producing a "mirrored set"; at least two drives are required to constitute such an array. While more constituent drives may be employed, many implementations deal with a maximum of only two; of course, it might be possible to use such a limited level 1 RAID itself as a constituent of a level 1 RAID, effectively masking the limitation.[citation needed] The array continues to operate as long as at least one drive is functioning. With appropriate operating system support, there can be increased read performance, and only a minimal write performance reduction; implementing RAID 1 with a separate controller for each drive in order to perform simultaneous reads (and writes) is sometimes called multiplexing (or duplexing when there are only two drives). WARNING: RAID 1 is not necessarily safe. In PC systems most HDD are shipped by manufacturers with "write cacheing" turned on; this gives an illusion of higher performance but at the risk of data not being written; failure for some reason (eg. power) can leave the two disks in an inconsistent state and even make them unrecoverable.

Pros – Guard against disk failure as data is replicated across disk drives
Cons – Replication creates storage overhead as the same data is copied across drives
Minimum Disks Required – 2 

In RAID 2 (bit-level striping with dedicated Hamming-code parity), all disk spindle rotation is synchronized, and data is striped such that each sequential bit is on a different drive. Hamming-code parity is calculated across corresponding bits and stored on at least one parity drive.

In RAID 3 (byte-level striping with dedicated parity), all disk spindle rotation is synchronized, and data is striped so each sequential byte is on a different drive. Parity is calculated across corresponding bytes and stored on a dedicated parity drive.

RAID 4 (block-level striping with dedicated parity) is identical to RAID 5, but confines all parity data to a single drive. In this setup, files may be distributed between multiple drives. Each drive operates independently, allowing I/O requests to be performed in parallel. However, the use of a dedicated parity drive could create a performance bottleneck; because the parity data must be written to a single, dedicated parity drive for each block of non-parity data, the overall write performance may depend a great deal on the performance of this parity drive.

Pros – Reduced storage overhead (actually we need only a single disk here to store parity). E.g. if you have 3 disks, parity can be stored on 3rd. So your overhead is only 33% in terms of storage.
Cons – Still suffers from a performance perspective
Minimum Disks Required – 3 (anyways 2 doesn’t make sense and the more no. of disks you have lesser would be your storage overhead)

RAID 5 (block-level striping with distributed parity) distributes parity along with the data and requires all drives but one to be present to operate; the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. However, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced and the associated data rebuilt. Additionally, there is the potentially disastrous RAID 5 write hole. RAID 5 requires at least three disks.

Pros – Good Performance, Good failure protection
Cons – Not as good when your requirement is only performance or only failure protection (parity doesn’t come for free).
Minimum Disks Required – 3

 RAID 6 (block-level striping with double distributed parity) provides fault tolerance of two drive failures; the array continues to operate with up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems. This becomes increasingly important as large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single-parity RAID levels are as vulnerable to data loss as a RAID 0 array until the failed drive is replaced and its data rebuilt; the larger the drive, the longer the rebuild takes. Double parity gives additional time to rebuild the array without the data being at risk if a single additional drive fails before the rebuild is complete. Like RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced and the associated data rebuilt.

RAID Comparison

RAID level	Min disks	Available storage capacity (%)	Read performance	Write performance	Write penalty	Protection
1	2	50	Better than  single disk	Slower than single disk, because every write must be committed to all disks	Moderate	Mirror
1+0	4	50	Good	Good	Moderate	Mirror
3	3	[(n-1)/n]*100	Fair for random reads and  good for sequential reads	Poor to fair for small random writes fair for large, sequential writes	High	Parity (Supports single disk failure)
5	3	[(n-1)/n]*100	Good for random  and sequential reads	Fair for random and sequential writes	High	Parity (Supports single disk failure)
6	4	[(n-2)/n]*100	Good for random and sequential reads	Poor to fair for random and sequential writes	Very High	Parity (Supports two disk failures)

 where n = number of disks

  

RAID 0+1: striped sets in a mirrored set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity.

The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror the data on the RAID system is lost.

    RAID 1+0: (a.k.a. RAID 10) mirrored sets in a striped set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity.

The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses so long as no mirror loses all its drives.

Pros: Best in terms of performance & guards against potential failures.
Cons: Costly in terms of storage overhead
Minimum Disks Required – 4 (and for > 4 you must have even number of disks)

    RAID 5+3: mirrored striped set with distributed parity (some manufacturers label this as RAID 53).

Whether an array runs as RAID 0+1 or RAID 1+0 in practice is often determined by the evolution of the storage system. A RAID controller might support upgrading a RAID 1 array to a RAID 1+0 array on the fly, but require a lengthy off-line rebuild to upgrade from RAID 1 to RAID 0+1. With nested arrays, sometimes the path of least disruption prevails over achieving the preferred configuration.

•       In RAID 5, every write (update) to a disk manifests as four I/O operations (2 disk reads and 2 disk writes)

•       In RAID 6, every write (update) to a disk manifests as six I/O operations (3 disk reads and 3 disk writes)

•       In RAID 1, every write manifests as two I/O operations (2 disk writes)

Source : http://en.wikipedia.org/wiki/RAID