Cheap and Reliable RAID 5 Storage Compared

When increased data security is required, administrators face some tough decisions. A RAID 1 (mirroring) solution is the simplest solution in many cases. It can never be a cut and dried decision, however, since RAID 1 offers few speed benefits over a single drive. Not only that, but sacrificing half of the available disk space may not prove universally popular.

An increasing number of decision-makers in smaller organizations are thus going down the RAID 5 road using ATA hard disks. The reasoning is that the hard disks are still considerably cheaper than SCSI equivalents - and the controllers are also reasonably priced. Not only that, but their performance is often not far behind the considerably more expensive SCSI solution.

We look at this new, attractive, mid-priced RAID category and subject five products that are representative of this category to our usual battery of lab tests: the 2410SA from Adaptec, HighPoint's RocketRAID 1640, the ICP Vortex 8545RZ, the MegaRAID 150-4 from LSI Logic, and the FastTrak S150 SX4 from Promise.

RAID 5: Why And For Whom?

Unlike the now well-established RAID levels 0 and 1, which offer either faster performance or increased security in the event of hard disk failure, RAID 5 offers both performance and security benefits. RAID 5 needs some powerful logic processing capabilities to control the simultaneous operation of several hard disks and to write data and parity information across all disks in the stripe set. The latter function does not require a particularly complex architecture, but as transfer rates increase, the process of writing parity data on the fly increases CPU overhead accordingly.

We must differentiate here between software RAID, as employed by HighPoint, and hardware RAID, in which a RISC processor carries out all processor-intensive operations. Promise has included its own XOR processor in its controller; Adaptec, ICP and LSI Logic prefer to rely on tried and true chips from Intel.

To write data and parity blocks across all the drives in the array, a RAID 5 setup requires a minimum of three hard drives. The "wasted" disk space in this case would be 33%, which is still rather high. The space required to store parity information decreases as the number of drives is increased, although the risk of drive failure is increased. The worst-case scenario is, of course, when two hard disks fail at the same time.

We can draw the following conclusions from the above: With four drives, you "sacrifice" ¼ of the available storage space, while maintaining a low likelihood of the worst-case scenario taking place. And a four-drive RAID 5 array is not restricted by the performance limits of the PCI bus with its 133 Mbps (32 bit, 33 MHz).

It is worth mentioning here that a RAID 5 array is hardly ever a viable solution for a desktop PC. Even with workstations, RAID 0+1 (striping + mirroring) or RAID 10 (mirroring + striping) remain the superior and fastest solutions, even though they utilize more of your available hard disk capacity.

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