Do RAID levels still matter? - Storage Technology Magazine - Page 1

Most new arrays stripe data across their spindles automatically to increase performance and better use disk capacity. With capabilities like that, RAID could become a thing of the past.


The 20-year anniversary of the invention of RAID by David Patterson, Garth Gibson and Randy Katz of the University of California at Berkeley is less than a year away. Their revolutionary paper, A Case for Redundant Arrays of Inexpensive Disks (RAID), changed the way server-class computers stored data. Soon after RAID burst upon the scene, storage administrators had to wrestle with the pivotal RAID question: "How shall I place data on my hard disks to optimize capacity, performance and data protection?" But that question is becoming less relevant because most new storage arrays automatically distribute data onto a number of spindles, which eliminates the manual task of selecting RAID levels.

Most Storage readers don't require an introduction to the concepts of RAID. But the rules of the game are changing. As recently as five years ago, storage administrators were constantly challenged by die-hard application administrators to control data placement on the disk array at a very granular level. Not being content with merely specifying "table spaces on RAID 5 and logs on RAID 0/1," some database administrators asked for particular data stripe placement on the platter itself

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so that, for example, the highest usage online transaction processing (OLTP) instances could get better performance by being written to volumes occupying the outermost disk cylinders.

Why RAID?
First and foremost, RAID was invented for increased storage performance. In essence, RAID is a form of parallel I/O processing that spreads the workload over a number of disk devices, summing their performance in an attempt to help storage keep up with the rest of the system. RAID does indeed achieve this goal, especially when used in conjunction with another powerful performance-enhancement mechanism: caching.

Caches keep getting larger, improving performance as they grow. Only when your application seeks data outside the cache (a cache "miss"), does the selection of RAID level affect performance. With new cache algorithms and proper tuning, cache misses can be kept to a minimum. With 90% plus cache "hits," RAID level selection will have a relatively minor impact. This hasn't gone unnoticed by storage array vendors.

This was first published in November 2007