RAID Levels - aryanjoshi0823/5143-Operating-System GitHub Wiki

Redundant Array of Independent Disks (RAID) is a data storage technology that uses multiple hard drives or SSDs to enhance fault tolerance, performance, or both. Commonly used in enterprise environments and by individuals, RAID protects data against hardware failures by mirroring data across drives or using parity for data recovery.

RAID systems consist of two or more drives working in parallel and can be implemented through hardware controllers or software. These systems support various interfaces like SATA, SCSI, and Fiber Channel. Operating systems such as Windows and macOS often provide software RAID functionality.
RAID levels, tailored for specific needs, range from simple striping for performance (RAID 0) to advanced redundancy methods like mirroring (RAID 1) or parity distribution (RAID 5/6). Drives not configured in RAID are called JBOD (Just a Bunch of Disks), acting independently and often used for non-critical data storage.

How Does RAID Work?

RAID combines multiple physical disks into a single logical unit using hardware or software. Commonly implemented on servers and high-performance workstations, RAID improves performance and fault tolerance by distributing data across drives.
RAID uses techniques like mirroring, which duplicates data across disks for redundancy, and striping, which spreads data across disks to enhance speed. Data is organized into blocks (stripes) and interleaved for optimized access. Small stripes work best for single-user systems, while larger stripes suit multiuser systems for overlapping I/O operations. RAID arrays appear as a single logical drive to the operating system, simplifying management.

RAID Levels:

Devices with a redundant array of inexpensive disks adopt different versions called levels. The original blueprint that brought about the term and developed the RAID setup enumerated several RAID levels. With these numbered systems, IT professionals could easily differentiate RAID versions. Recently, the number of RAID levels has been categorized into three groups:

a)Standard RAID Levels:

1. RAID 0:

RAID 0 simply entails merging multiple disks into a single volume. This augments the speed of operation as users are simultaneously reading and writing from multiple disks. A single file can then use the speed and capacity of the entire drive.
The demerit of RAID 0 is that it lacks redundancy. If a single disk is lost, there will be complete data loss. It is not advisable to use RAID 0 in a server environment is not advisable. Still, it can be used for other purposes where speed is vital and data loss doesn’t cause significant havoc, such as cache.

2. RAID 1:

RAID 1 uses mirroring. Compared to RAID 0, RAID 1 can carry out more sophisticated configurations. The most common use case of RAID 1 is where users possess a pair of similar disks that identically replicate the data across the entire drives in the array.
The primary objective of RAID 1 is redundancy. If users lose a drive, additional drives will keep on running the operation. Also, if there’s a drive failure, users can replace the faulty drives without any downtime. Furthermore, RAID 1 provides users with better read performance. As a result, data can be read off on any of the drives present in the array. Nevertheless, this comes with a downside which is a slightly higher write latency. This is because users need to write data on both drives in the array and only the capacity of a single drive is available.

2. RAID 2:

Generally, RAID 2 is rarely used practically. RAID 2 stripes data at the bit level and uses a Hamming code to rectify errors. The disks in RAID 3 are synchronized by the controller, which causes them to spin at corresponding angles such that they attain index points at the same time. Therefore, RAID 2 cannot efficiently handle multiple requests at the same time. Notwithstanding, contingent upon the rate of the Hamming code, several spindles would operate in parallel to ensure a simultaneous transfer of data, such that very high data transfer rates are feasible.
Since error corrections are implemented on all hard disk drives, the complexity of an external Hamming code offers an advantage over uniformity. For this reason, RAID 2 has been infrequently implemented, and it is the only standard RAID level that is currently unused.

RAID 3:

RAID 3 entails byte-level striping with a committed parity disk. Among the features of RAID 3 is that it can not effectively monitor multiple requests simultaneously. The reason for this is that any single block of data will be transmitted across all the entire set members and will occupy the exact physical location on each disk. Therefore, any input/output operation will require activity over the whole disks, as well as synchronized spindles.
For these reasons, RAID 3 is suitable for applications that require the highest transfer rates in long chronological reads and write. This RAID level will perform woefully for applications that make miniature reads and writes from random disk locations.

RAID 4:

RAID 4 entails block-level striping with a dedicated parity disk. The layout of RAID 4 provides good performance of random reads even though the performance of random writes is low because of the need to write the entire parity data to a single disk. This can be taken care of if the filesystem is RAID-4-aware and compensates for that.
RAID has the edge over others because it can be quickly extended online. This doesn’t require parity recomputation, as long as the newly added disks are filled with 0-bytes.

RAID 5 and 6:

RAID 5 and RAID 6 use similar techniques. For RAID 5 to be used, there must be at least three drives. On the other hand, RAID 6 requires at least four drives. This level incorporates the idea of RAID 0 and stripes data across multiple drives to augment performance.
However, it also adjoins the aspect of redundancy by distributing parity information across the disks. In essence, RAID 5 can lose only one disk and maintain operations without interruption. RAID 6 can lose two disks and still maintain operations and data without any hitches. RAID 5 and 6 provide users with better read performance. However, the write performance is contingent upon the utilized RAID controller.
RAID 5 and 6 require a dedicated hardware controller because of the need to compute the parity data and write it across the entire disk. Hence, RAID 5 and 6 are suitable options for file servers, standard web servers, and other systems where most of the transactions are read.

b) Nested RAID Levels:

Nested RAID levels are obtained from the combination of standard RAID levels. Some examples of nested RAID levels are:

RAID 10 (RAID 1+0):

When RAID 1 and RAID 0 are combined, RAID 10 is produced. RAID 10 is more expensive than RAID 1 and also offers better performance than RAID 1. The data in RAID 10 is mirrored, and the mirrors in this RAID are striped.

RAID 03:

RAID 0+3, otherwise known as RAID 53 or RAID 5+3, adopts RAID 0’s striping method and RAID 3’s virtual disk blocks. This produces a higher performance compared to RAID 3, but at a higher cost.

c) Non-Standard RAID levels:

Non-Standard RAID levels differ from standard RAID levels and are usually developed by companies primarily for exclusive use. Some examples of non-standard RAID levels are:

RAID 7:

RAID 7 is a non-standard RAID level obtained from RAID 3 and RAID 4. RAID 7 entails caching via a high-speed bus, real-time incorporated operating system as a controller, and other features of a solitary computer.

d) Adaptive RAID: Adaptive RAID allows the RAID controller to determine how the parity on disks will be stored. Adaptive RAID chooses between RAID 3 and RAID 5.

RAID and Data Backup and Recovery:

RAID is often used in conjunction with data backup and recovery strategies to help protect against data loss.
RAID arrays can provide redundancy and fault tolerance, which means that if one or more hard drives fail, data can still be accessed and recovered from the remaining disks in the array. However, RAID is not a replacement for a proper backup strategy. While RAID can protect against hardware failures, it cannot protect against data loss due to software issues, human error, or other factors.
It’s important to have a separate backup strategy in place that includes regular backups of important data to an external location or cloud-based storage. This ensures that if data is lost or corrupted for any reason, it can be easily restored from the backup