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The Sky's The Limit: Nested RAID

IDE Training Course, Part 3: Using RAID
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If the data transfer rate of an array with several drives is still not enough, you can combine and nest RAID arrays any way you like. These configurations are called Nested RAID (multiple RAID levels), but you'll encounter them very rarely - and no wonder, because "conventional" arrays are generally fast enough.

As far as we know, controllers supporting Nested RAID are not yet available in the IDE sector (with the exception of RAID 10). For SCSI, be prepared to quickly dish out several hundred to some thousand dollars if you want to set up an elaborate RAID solution.

RAID Level 0+1

The most popular Nested Raid is probably 0+1. You'll need an even number of hard drives for this, but at least four. Use half of the hard drives to create a stripe set (RAID 0), while the resulting construct is simply mirrored (with RAID 1). You will then get almost four times the read performance and about twice the write performance relative to a single hard drive.

RAID Level 50 (5+0)

The performance of a RAID 5 with several drives is not good enough for you? Then simply create a stripe set consisting of two identical RAID 5 arrays. Though data security is no longer a given now (an array is to be considered a drive in this case), performance can theoretically be doubled once more. In reality, you'll now be faced with the limits of what PCI and network connections will allow.

Naming is an important factor in Multiple or Nested RAID configurations. While RAID 0+1 works on the lower level with stripe sets and mirrors only on the upper level, with RAID 10 it's exactly the opposite. As the latter does not really make sense, the wrong nomenclature would be less grave in this case.

Nested RAID And Security: It's All Or Nothing

Now, a few words on the cascaded application of RAID arrays, even though most of you will probably never be in a situation to have to worry about linked drives like this.

Combining several RAID arrays is efficient and prudent, but perfect data security can be achieved only if each array is just as safe in and of itself. A RAID 5 consisting of multiple RAID 0 arrays is not secure, because if one of the drives of a secondary array crashes, its data cannot be recovered.

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