RAID stands for redundant array of inexpensive disks or redundant array of independent disks. It is a data storage virtualization technology that provides a way of storing the same data in different places on multiple hard disks. It improves the performance by placing the data on multiple disks. The input/output (I/O) operations can overlap in a balanced way and it reduces the risk of losing all data if one drive fails. RAID storage uses multiple disks in order to provide fault tolerance and it increases the storage capacity of the system. These disks can be combined into different array configurations known as RAID levels. Raid levels has its own characteristics such as fault-tolerance, performance and capacity. Fault-tolerance is the ability to survive one or several disk failures. Performance shows the change in the read and write speed of the entire array as compared to a single disk. The capacity of the array is determined by the amount of user data that can be written to the array. RAID can be used in large file servers, transaction of application servers where data accessibility is critical, and fault tolerance is required. RAID is an emerging storage technology with the potential to remodel the data storage technology. A typical RAID unit contains a set of disk drives, which is equivalent to a single large capacity disk drive.
RAID storage techniques
The main data storage methods in the array are:
1) Striping: In this storage technique it splits the flow of data into blocks of a certain size called block size then these blocks are written across the RAID one by one. This type of data storage effects the performance.
2) Mirroring: In this storage technique the identical copies of data are stored on the RAID members simultaneously. This way of data storage affects the fault tolerance as well as the performance.
3) Parity: In this storage technique it uses striping and checksum methods. A certain parity function is calculated for the data blocks. The missing blocks can be recalculated from the checksum, providing the RAID fault tolerance.
1) RAID 0: Striped disk array without fault tolerance. RAID 0 provides great performance, both in read and write operations. There is no overhead caused by parity controls and this technology is easy to implement.
2) RAID 1: (Mirroring and duplexing) RAID 1 provides excellent read speed and a write-speed that is comparable to that of a single drive. RAID 1 is a very simple technology and if a drive fails, data is copied to the replacement drive.
3) RAID 2: (Error-correcting coding) RAID 2 is rarely used and is similar to RAID 5. Instead of disk striping using parity, striping occurs at the bit-level. In RAID 2 it stripes data at the bit (rather than block) level, and uses a Hamming code for error correction. It is the only original level of RAID that is not currently used.
4) RAID 3: (Bit-interleaved parity) It consists of byte-level striping with a dedicated parity disk. It performs poorly when there are a lot of little requests for data, as in a database, so this RAID type is not currently used.
5) RAID 4: (Dedicated parity drive) RAID 4 is good for sequential data access and provides good performance of random reads, while the performance of random writes is low due to the need to write all parity data to a single disk. It consists of block-level striping with a dedicated parity disk.
6) RAID 5: (Block interleaved distributed parity) RAID 5 provides excellent Performance and Full Fault Tolerance. It can be rebuilt from Parity information from all drives and used in file servers, web servers, and important backups. Read data transactions are very fast while write data transactions are somewhat slower (due to the parity that has to be calculated).
7) RAID 6: (Independent data disks with double parity) RAID 6 provides full Fault tolerance. It is a large, reliable, relatively expensive storage and uses a block pattern similar to RAID5, but utilizes two different parity functions to derive two different parity blocks per row.
8) RAID 10: (A stripe of mirrors) RAID 10 is a combination of RAID 1 and 0. It is often denoted as RAID 1+0. It combines the mirroring of RAID 1 with the striping of RAID 0. RAID 10 provide good read and write performance. It can be used in Database storage for high performance and availability.
Benefits of RAID
1) Higher Data Security
2) Fault Tolerance
3) Increase the parity check and regularly checks for any possible system crash
4) Reading and Writing of data done at simultaneously.
5) Improved Availability and performance.
6) Ensures data reliability
Disadvantages of RAID
1) It doesn’t make data recovery any easier.
2) Cannot completely protect your data.
4) It may slow the system performance if not used properly
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