RAID Technology

What is RAID?

RAID stands for Redundant Array of Independent Disks. To provide data protection against drive failures RAID technology leverages multiple drives as part of a set. In general, by serving I/Os from multiple disks simultaneously RAID implementations also improve the storage system performance. By using RAID modern arrays with flash drives also benefit in terms of protection and performance.

Why use RAID?

There’s always a risk of data loss due to disk failure, having redundancy is very important to avoid any disastrous consequences. By using this technology in your storage system you can achieve good redundancy and a good amount of read-write speed.

RAID Techniques:-

• Striping: In striping, technique data is spread across multiple drives (more than one) to use the drives in parallel. More data is processed in a shorter time when all the read-write heads work simultaneously and it increases performance, compared to reading and writing from a single disk.

• Mirroring: In the mirroring technique the same data is stored on two different disk drives, giving two copies of the data. In case of a single disk drive failure, the data present is intact on the surviving disk drive and the controller remains servicing the host’s data requests from the surviving disk of a mirrored pair.

• Parity: Parity is a technique to protect striped data from disk drive failure without the cost of mirroring. To hold parity an additional disk drive is added, a mathematical construct that allows re-creation of the missing data. Parity is a redundancy technique that doesn’t maintain a full set of duplicate data and ensures the protection of data. RAID controller has the function of calculating parity.

RAID LEVELS:

• RAID 0: RAID 0 configuration uses data striping techniques, where data is striped across all the disks within a RAID set. Therefore it utilizes the full storage capacity of a RAID set. All the strips are put back together by the controller to read data,. It has no redundancy i.e. zero data protection in case of storage failure but this method has a good read write speed.

• RAID 1: RAID 1 is based on the mirroring technique. In this RAID configuration, to provide fault tolerance data is mirrored. In a RAID 1 set, there are two disk drives and every write is written to both disks. The mirroring is transparent to the host. Among all RAID implementations during disk failure, the impact on data recovery in RAID 1 is the least. This is because for data recovery the RAID controller uses the mirror drive. When applications that require high availability and cost is no constraint RAID 1 is suitable for it.

• NESTED RAID: From their RAID arrays most data centers require data redundancy and performance. RAID 1+0 and RAID 0+1combine the performance benefits of RAID 0 with the redundancy benefits of RAID 1. They combine their benefits using striping and mirroring techniques. The minimum number of disks is four and these types of RAID require an even number of disks.

• RAID 3/4: RAID 3 stripes data for performance and uses parity for fault tolerance. If a drive fails in a RAID set, parity information is stored on a dedicated drive so that the data can be reconstructed. For example, in a set of five disks, one is used for parity and four are used for data. Therefore, we need is 1.25 times the total disk space of the size of the data disks. As the drives operate in parallel RAID 3 always reads and writes complete stripes of data across all disks.

CONCLUSION:

Individual disks are prone to failures and pose the threat of data unavailability. By using mirroring and parity techniques RAID addresses data availability requirements. In addition to redundancy benefits RAID implementations with striping enhance I/O performance by spreading data across multiple disk drives. The selection of a RAID level depends on the performance, cost, and data protection requirements of an application.