Striping/Mirroring & Spanning

RAID 0 and RAID 1 can be combined into a function known as, you guessed it, RAID 0/1 which allows for a minimum of two drives to act as mirrors to two other drives that act as striped pairs. Using this technology, you receive the performance benefit from RAID 0 while receiving the fault tolerance protection of RAID 1. The downside to RAID 0/1 is the cost, due to the fact that a total minimum of four drives must be present to take advantage of the technology. The capacity of a RAID 0/1 system is calculated in the same manner as a RAID 1 system, which is an array equal in capacity to that of the smallest drive in the array multiplied by the number of drives, divided by two due to the fault tolerance support (half of the drives are mirrored by the other half).

It is quite simple to envision a RAID system consisting of two disks, running in level 0, level 1, or as a hybrid in 0/1, however what happens when you have four disks (RAID generally is setup with even numbers of drives)? In the case where more than one drive is to be used as a primary drive, a relatively common technology known as spanning is used, where two or more drives function as if they were one large drive.

Although there are many more levels of RAID that are present in more advanced setups, the basics of RAID have already been outlined, and it is upon those three levels of RAID (0, 1, and 0/1) that Promise based their IDE RAID controller, the FastTrack.

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