While both the processor and chipset companies viciously fight for market supremacy, other parts of the hardware sector inevitably move to the background. Intel's Pentium 4 and AMD's Athlon XP processor continue the struggle for the performance crown, and the chipset arena is as colorful as never before - by comparison, the situation amongst storage controllers receives little hype from the media and is often forgotten as a consequence. RAID (Redundant Array Of Independent Disk Drives) has become a powerful technology to speed up disk subsystem performance and/or to increase data safety.
The progress in developing IDE RAID controllers has generated powerful devices with a useful range of features. In addition, companies like AMI and Promise offer more sophisticated controllers that support even more drives (Promise FastTrak100 TX4, 4 IDE channels) or more complex RAID modes (RAID 5: Promise SuperTrak SX6000). IDE drives have always been more attractive if you consider price to be the most important factor. In addition, at the same rotation speed, they are not any slower than SCSI models in term of data transfer performance or access time. High-end IDE controllers might not be much cheaper than SCSI hardware, but the drives themselves usually cost only a third of the price! That's why IDE RAID 3 and 5 is likely to replace SCSI RAID setups in systems that do not require 100% perfection, but merely 100% function.
In this respect, the three RAID chips from AMI, HighPoint and Promise can be seen as harbingers of the future of IDE - let's learn a bit more about them now!
Available RAID Levels
First of all we should distinguish between "real" RAID modes and those you have to cut back for. Common RAID Levels are 0, 1, 0+1, 3 and 5. Only the latter two speed up performance while simultaneously enhancing data safety.
Many people do not consider Level 0 a real RAID mode, as it is risky in terms of data safety. RAID 0 does more than store data across two or more drives in so-called stripes (pretty much like a zipper). The idea behind this is to merge the capacity of all drives while increasing performance by distributing data over all drives, so that it can be read in parallel.
However, there is no fault-tolerance, as one defective drive will result in the complete loss of all data.
Mode 1 is the counterpart to Mode 0 and intended to increase data safety. Here, all data is simply mirrored to a second drive.
It is not faster than a single drive, but provides excellent data safety. Even if one drive crashes, the system will continue working.
This mode combines the two modes that I already described. First, take two drives and build a stripeset. This stripeset will be written simultaneously onto an additional array that will continue running if the primary one should fail.
RAID 3, 5
RAID Levels 3 and 5 are usually available with SCSI RAID adapters only. Starting at three drives, high-end RAID controllers use intelligent algorithms with checksums to distribute data on all drives that are available. RAID 3 uses one drive to store parity information, while RAID 5 stores that information alternating to all drives, offering even better performance. RAID Level 5 has become the dominant mode for critical servers. For more information on this topic please read RAID Overview at the IBM website. They have useful illustrations of RAID mode 5.