High Availability Storage (I)

RAID Concepts

The acronym RAID stands for Redundant Array of Inexpensive Disks and is a technology that provides increased storage functions and reliability through redundancy. It was developed using a large number of low cost hard drives linked together to form a single large capacity storage device that offered superior performance, storage capacity and reliability over older storage systems. This was achieved by combining multiple disk drive components into a logical unit, where data was distributed across the drives in one of several ways called "RAID levels".
This concept of storage virtualization and was first defined as Redundant Arrays of Inexpensive Disks but the term later evolved into Redundant Array of Independent Disks as a means of dissociating a low-cost expectation from RAID technology.
There are two primary reasons that RAID was implemented:

• Redundancy: This is the most important factor in the development of RAID for server environments. A typical RAID system will assure some level of fault tolerance by providing real time data recovery with uninterrupted access when hard drive fails;

• Increased Performance: The increased performance is only found when specific versions of the RAID are used. Performance will also be dependent upon the number of drives used in the array and the controller;

Hardware-based RAID

When using hardware RAID controllers, all algorithms are generated on the RAID controller board, thus freeing the server CPU. On a desktop system, a hardware RAID controller may be a PCI or PCIe expansion card or a component integrated into the motherboard. These are more robust and fault tolerant than software RAID but require a dedicated RAID controller to work.

Hardware implementations provide guaranteed performance, add no computational overhead to the host computer, and can support many operating systems; the controller simply presents the RAID array as another logical drive

Software-based RAID

Many operating systems provide functionality for implementing software based RAID systems where the OS generate the RAID algorithms using the server CPU. In fact the burden of RAID processing is borne by a host computer's central processing unit rather than the RAID controller itself which can severely limit the RAID performance.

Although cheap to implement it does not guarantee any kind of fault tolerance; should a server fail the whole RAID system is lost.

Hot spare drive

Both hardware and software RAIDs with redundancy may support the use of hot spare drives, a drive
physically installed in the array which is inactive until an active drive fails. The system then automatically replaces the failed drive with the spare, rebuilding the array with the spare drive included. This reduces the mean time to recovery (MTTR), but does not completely eliminate it. Subsequent additional failure(s) in the same RAID redundancy group before the array is fully rebuilt can result in data loss. Rebuilding can take several hours, especially on busy systems.

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