RAID in Linux & RAID Levels

RAID in Linux

The term RAID is an acronym for the phrase, Redundant Array of Independent Disks. RAID is a way of combining the storage available across multiple disks and supplying users a single, unified virtual device.
RAID can be used to provide:

• data integrity
• fault tolerance
• improved performance
• greater storage capacity

Hard disks are mechanical devices involving moving parts and unfortunately tend to fail over time. There are also physical limits to the speed at which data can be read and/or written to disks. RAID helps mitigate this risk by protecting data stored on hard disks and improving disk performance by writing the data to multiple

Physical locations according to several different schemas, known as "RAID Levels". Furthermore, RAID can be provided by either dedicated, specialized hardware or by the operating system at a virtual layer.

Hardware RAID solutions exist that operate as dedicated devices, usually as PCI expansion cards or directly on the motherboard. The independent disks attach to the hardware interface. In a true hardware RAID, the operating system simply writes data to the hardware RAID controller which handles the multiplicitous reads and writes to the associated disks. Other so−called hardware RAIDs rely on special drivers to the operating system; these act more like software RAIDs in practice. With current technology, hardware RAIDconfigurations are generally chosen for very large RAIDs.

Additionally, some operating systems, including Linux®, provide RAID functionality within a software layer.RAID partitions are logically combined and a virtual device appears to higher layers of the operating system in place of the multiple constituent devices. This solution is often a high−performance and inexpensive alternative available for RAID users.

RAID levels

There are many RAID levels. It will be impossible to list them all here. In this classroom, we will mention the most common, most important RAID types, all of which are fully supported by Linux.

RAID 0 (Striping)

This level is achieved by grouping 2 or more hard disks into a single unit with the total size equaling that of all disks used.

Practical example: 3 disks, each 80GB in size can be used in a 240GB RAID 0 configuration.

RAID 0 works by breaking data into fragments and writing to all disk simultaneously. This significantly improves the read and write performance.

On the other hand, no single disk contains the entire information for any bit of data committed. This means that if one of the disks fails, the entire RAID is rendered inoperable, with unrecoverable loss of data.

RAID 0 is suitable for non-critical operations that require good performance, like the system partition or the /tmp partition where lots of temporary data is constantly written. It is not suitable for data storage. 

Uasable Space in Raid level0 = (smallest disk) * (no. of disks)