And, by the way, just exactly what do we want from RAID?

In general, two things: high speed or high reliability. And, if we can, both of them.

We can acheive high speed in some variants of RAID by setting in-out operations in two parallel modes to several different disk devices. We can increase reliability because several kinds of RAID keep track of additional information that helps to restore data after system crash.

For example, assume we need a "fast" RAID system. First, it should be noted that RAID can parallelize data streams for physical devices only, so paritions in "fast" RAID systems need to be on different hard drives. If you are using IDE-RAID be sure to remove all slave devices! Any one of these devices will slow down data exchange for other devices because in IDE it's impossible to maintain different data exchange rates with both devices on one cable.

For "reliable" RAID systems, you need to remember the above mentioned IDE-RAID caveat too, though for another reason. Even if you have SCSI or some other type of device, don't place too many devices on one interface. For example, in the case of a 40 Mbit UW-interface, with hard drives that each support data streams of 10-12Mbit, we don't need to place more than 3-4 such disks on that cable.

Let's discuss "reliable" RAID some more, and just what that term means. You should never think that software RAID will protect you from all software problems and errors or will eliminate the neccessity of performing a backup of your system. Nothing could be further from the truth. Any RAID is a low-level function, and any data corruption done by the system will be invisible to the RAID functions and will be duplicated on additional hard stores. The same holds true about any kind of disk errors, which cannot be detected by controller, either.

You also shouldn't try to use RAID in place of an APC reserve-power device--once the electricity is off, some data exchange transactions on the disks could be in different stages of completion, and after the next reload, the array will be asynchronized. To minimize the probability of such trouble, some hardware RAIDs can be integrated with reserve power batteries.

So basically, here's what you should know: "reliable" RAID can help you to keep your data safe only in case of good disk hardware error detection, which depends on the "level" of RAID--something we will discuss in the next section.

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