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RAID 15 And RAID 1.5 In Detail

RAID 1.5 With IDE: Added Value Or Eyewash?
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First things first: HighPoint does not operate "true" RAID 15. As its name indicates, a RAID 15 array is made up of several logical drives, each of which is comprised of RAID 1 arrays. This is called a "nested" RAID setup:

RAID 15 is made up of at least three logical elements (the minimum requirement for RAID 5) that are in turn comprised of RAID 1 arrays. RAID 51 is exactly the opposite: it mirrors two RAID 5 arrays.

It's not difficult to see that combining two RAID modes greatly enhances data security. With a RAID 15 array, one drive can crash in each RAID 1 block without bringing the whole system to its knees.

So HighPoint should be aiming for something similar if it wants to hawk its product with the RAID 15 label. The manual for DFI's LANParty 875 Pro and LANParty KT400A motherboards contains the following table:

This makes it abundantly clear that RAID 1.5 offers optimum performance and data security (oval on the right) - without having anything to do with real RAID 15. So far, so good. What doesn't quite cut it, though, is the claim that it supports both striping and mirroring. The answer to this question is simply the dot between the 1 and the 5, as we are talking about RAID 1.5. Both RAID 1.5 and RAID 15 does nothing else than combining striping (read access over two drives simultaneously) and mirroring (data is written like in RAID 1).

In fact, although the controller does handle physical striping - i.e., data are alternately written (or read) to one disk and then the other, maximizing the data stream because both drives are being used - that's still not enough: it would basically be nothing more than plain old RAID 0. The capacity available with the RAID 1.5 array equals the capacity of a single hard drive, leaving the other half of the total capacity for parity data."

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    lifeisstudy , May 5, 2008 6:14 AM
    So RAID 1.5 won't work because the heads of the disk still need to move to the next data block to read... If you skip blocks, you still have to move past them.

    What about RAID 1.5 with Solid State disks. You have nothing to move, yet you can double your read speed with mirroring. Sounds perfect for server caches.