double-parity RAID

In a conventional RAID, data is stored in different places on multiple hard disks, thereby increasing the aggregate mean time between failures (MTBF) and improving the fault-tolerance. In the approach known as RAID-4, the number of bits in data blocks on multiple disks is added up, and the total is kept on a disk called the parity disk. If a drive fails, data recovery is facilitated by using the bits stored on the parity disk and bits remaining on the surviving drives. In RAID-5, the parity information is stored diagonally across all the disks in the RAID set. If a single drive fails, the original data is calculated from the parity information remaining on the surviving disks in the set.

Because hard disk storage capacity continues to grow, and because RAID systems are using larger numbers of disks, the risk of a double-disk hit (failure of two disks at the same time) is increasing. In a conventional RAID, such an event can result in extended downtime. In a double-parity RAID, this risk is minimized by the use of two parity disks. The number of bits is added up in a redundant fashion, and two sets of parity information is stored diagonally across rows of data blocks. As a result, data can be recovered easily, even if two hard disks fail simultaneously.

RAID-6 (double-parity RAID) systems are offered by several vendors. The main advantage is enhanced data protection as compared with single-parity RAID systems. Some engineers estimate that the MTBF in a double-parity RAID is over 100 times the MTBF in a single-parity RAID. Conversion from a conventional RAID to a double-parity RAID is said to be straightforward and not overly expensive. Though double parity RAID exacts more of a performance penalty than RAID-5, it is said to offer twice the protection. Double-parity RAID also allows up to 56 physical drives (a minimum of four drives are required), offering an advantage in logical unit (LUN) sizes from a single RAID-set over RAID-5 implementations, which are typically 16 drives or less.

Learn more about RAID

Post-RAID products: Some industry observers believe the scale of storage has outpaced RAID's usefulness as a data protection mechanism.
RAID 6 vs. RAID 10: Rick Cook outlines the most efficient uses of RAID 6 and RAID 10.
Learn all about RAID storage: This handbook describes the different levels of RAID technology and explains how storage administrators can choose the appropriate RAID level.
The evolution of RAID data protection: RAID has evolved in the last 20 years, but the core concept of combining drives to protect data lives on regardless of the methods used.
A failed RAID drive needs immediate attention: This tip emphasizes the importance of replacing a failed RAID drive immediately and the possible consequences of not doing so in a timely manner.
RAID data storage tutorial: RAID strategies for small- and medium-sized businesses (SMBs): In this tutorial on RAID data storage for SMBs, learn about RAID data protection is changing, how to choose the correct RAID level, and optimizing RAID storage for your business.
The state of MAID in data centers: MAID and spin-down storage systems are playing into green IT and consolidation trends. Regulations mandating more power savings will drive the technology, as will persistent data.
RAID technology overview: This RAID (redundant array of independent disks) guide offers fast info on the levels, implementation, boosting performance, maintenance and more.
	RAID FAQ: Storage analyst Greg Schulz answers common questions about RAID, such as which level of RAID to use.
	Disk Hardware FAQ: Greg Schulz, founder and senior analyst with StorageIO, offers answers to the most common disk hardware questions he hears in the field.

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More resources from around the web: - Network Appliance describes their double-parity RAID system (PDF, 252 KB). - PCGuide provides an overview of RAID technology. - Techworld discusses methods of protection against storage-media failure. - SearchStorage.com expert Christopher Poelker advises a user about double-parity RAID.

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