RAID Basics

RAID (Redundant Array of Independent Disks)

RAID or Redundant Array of Independent Disks, is a technology to connect multiple secondary storage devices and use them as a single media. RAID consists of an array of disks in which multiple disks are connected together to achieve different goals. RAID is a core feature of server hardware that ensures data integrity. Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the required level of redundancy and performance. RAID works by placing data on multiple disks and allowing input/output (I/O) operations to overlap in a balanced way, improving performance. Because the use of multiple disks increases the mean time between failures, storing data redundantly also increases fault tolerance. Mirroring provides high reliability, but it is expensive. Striping provides high data transfer rates, but does not improve reliability. Various alternative schemes aim to provide redundancy at lower cost by combining disk striping with “parity” bits (which we describe next). These schemes have different cost–performance trade-offs. The schemes are classified into RAID levels :

1. RAID 0 : In this level, a striped array of disks is implemented. The data is broken down into blocks and the blocks are distributed among disks. Each disk receives a block of data to write/read in parallel. It enhances the speed and performance of the storage device. There is no parity and backup in Level 0.

2. RAID 1 : Also known as disk mirroring, this configuration consists of at least two drives that duplicate the storage of data. There is no striping. Read performance is improved since either disk can be read at the same time. Write performance is the same as for single disk storage.

3. RAID 2 : This configuration uses striping across disks, with some disks storing error checking and correcting (ECC) information. The idea of error-correcting codes can be used directly in disk arrays by striping bytes across disks. For example, the first bit of each byte could be stored in disk 1, the second bit in disk 2, and so on until the eighth bit is stored in disk 8, and the error-correction bits are stored in further disks.

4. RAID 3 : RAID 3 stripes the data onto multiple disks. The parity bit generated for data word is stored on a different disk. This technique makes it to overcome single disk failures. bit-interleaved parity organization, improves on level 2 by exploiting the fact that disk controllers, unlike memory systems, can detect whether a sector has been read correctly, so a single parity bit can be used for error correction, as well as for detection. The idea is as follows. If one of the sectors gets damaged, the system knows exactly which sector it is, and, for each bit in the sector, the system can figure out whether it is a 1 or a 0 by computing the parity of the corresponding bits from sectors in the other disks. If
the parity of the remaining bits is equal to the stored parity, the missing bit is 0; otherwise, it is 1.

5. RAID 4 : In this level, an entire block of data is written onto data disks and then the parity is generated and stored on a different disk. This level uses large stripes, which means you can read records from any single drive. This allows you to use overlapped I/O for read operations. Since all write operations have to update the parity drive, no I/O overlapping is possible. Note that level 3 uses byte-level striping, whereas level 4 uses block-level striping.

6. RAID 5 : This level is based on block-level striping with parity. The parity information is striped across each drive, allowing the array to function even if one drive were to fail. The array's architecture allows read and write operations to span multiple drives. This results in performance that is usually better than that of a single drive, but not as high as that of a RAID 0 array.

7. RAID 6 : RAID 6 is an extension of level 5. RAID 6 consists of block-level striping with double distributed parity. Double parity provides fault tolerance up to two failed drives. In this level, two independent parities are generated and stored in distributed fashion among multiple disks. Two parities provide additional fault tolerance.

Nested RAID

Nested RAID levels, also known as hybrid RAID, combine two or more of the standard RAID levels to gain performance, additional redundancy or both, as a result of combining properties of different standard RAID layouts.

RAID 10 : RAID 10, also known as RAID 1+0, combines disk mirroring and disk striping to protect data. RAID 10 provides redundancy and performance, and is the best option for I/O-intensive applications. One disadvantage is that only 50% of the total raw capacity of the drives is usable due to mirroring.

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