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RAID: What it is and how it works

RAID is a method of creating one or more stores of data storage space from several hard drives. It can offer fault tolerance and higher throughput levels than a single hard drive or group of independent hard drives. You can build a RAID configuration with IDE (parallel ATA), SATA (Serial ATA) or SCSI hard disks. Dell is shipping computers pre-configured with RAID 0 arrays for high output gaming and video applications. The problem with RAID 0 is if one drives goes, that’s it! The data is lost. RAID 0 data recovery is the absolute toughest data recovery situation.

The exact meaning of RAID has been much debated and much argued. The use of “Redundant” is, in itself, a contentious point. That several manufacturers have deviated from accepted RAID terminology, created new levels of disk arrangements, called them RAID, and christened them with a number has not helped. There are even some single disk RAID configurations! Double parity, RAID 1.5, Matrix RAID etc., are examples of proprietary RAID configurations. One of the key players in NAS devices – SNAP has a very proprietary system of RAID. SNAP OS is also a unique operating system. While based on Free BSD, it is still a very complex recovery.

Data can be distributed across a RAID “array” using either hardware, software or a combination of the two. Hardware RAID is usually achieved either on-board on some server class motherboards or via an add-on card, using an ISA/PCI slot. Newer motherboards that use SATA technology have RAID controllers “on board”.

A RAID 0 (also known as a stripe set or striped volume) splits data evenly across two or more disks (striped) with no parity information for redundancy. It is important to note that RAID 0 was not one of the original RAID levels and provides zero data redundancy. RAID 0 is normally used to increase performance, although it can also be used as a way to create a small number of large virtual disks out of a large number of small physical ones.

A RAID 1 creates an exact copy (or mirror) of a set of data on two or more disks. This is useful when read performance or reliability are more important than data storage capacity. Such an array can only be as big as the smallest member disk. A classic RAID 1 mirrored pair contains two disks (see diagram), which increases reliability exponentially over a single disk. Since each member contains a complete copy of the data, and can be addressed independently, ordinary wear-and-tear reliability is raised by the power of the number of self-contained copies.

A RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5 has achieved popularity due to its low cost of redundancy. Generally, RAID 5 is implemented with hardware support for parity calculations. A minimum of 3 disks is generally required for a complete RAID 5 configuration (A RAID 5 two disk set is possible, but many implementations do not allow for this. In some implementations a degraded disk set can be made (3 disk set of which 2 are online). When it comes to RAID 5 Data Recovery, DTI only needs to recover 2 of the 3 drives (if it is a 3 disk set) to get the files back!

Many storage controllers allow RAID levels to be nested.

Common nested RAID levels:

RAID 01: A mirror of stripes
RAID 10: A stripe of mirrors
RAID 50: A stripe across dedicated parity RAID systems
RAID 51: A mirror striped set with distributed parity (some manufacturers label this as RAID 53)
RAID 100: A stripe of a stripe of mirrors

This entry was posted on Saturday, February 10th, 2007 at 7:00 am and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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