The single RAID levels have distinct advantages and
disadvantages, which is why most of them are used in various parts of the market to
address different application requirements. It wasn't long after RAID began to be
implemented that engineers looked at these RAID levels and began to wonder if it might be
possible to get some of the advantages of more than one RAID level by designing arrays
that use a combination of techniques. These RAID levels are called variously multiple,
nested, or multi-RAID levels. They are also sometimes called two-dimensional,
in reference to the two-dimensional schematics that are used to represent the application
of two RAID levels to a set of disks, as you shall see.
Multiple RAID levels are most commonly used to improve performance, and they do this
well. Nested RAID levels typically provide better performance characteristics than either
of the single RAID levels that comprise them. The most commonly combined level is RAID 0,
which is often mixed with redundant RAID levels such as 1, 3 or 5 to provide fault
tolerance while exploiting the performance advantages of RAID 0. There is never a
"free lunch", and so with multiple RAID levels what you pay is a cost in
complexity: many drives are required, management and maintenance are more involved, and
for some implementations a high-end RAID controller is required.
Not all combinations of RAID levels exist (which is good, because I'd get really bored
of describing them all! 
) Typically, the most popular multiple RAID levels are those
that combine single RAID levels that complement each other with different strengths and
weaknesses. Making a multiple RAID array marrying RAID 4 to RAID 5 wouldn't be the best
idea, since they are so similar to begin with.
In this section I take a look at some of the more common multiple RAID levels. Note
that some of the multiple RAID levels discussed here are frequently used, but others are
rarely implemented. In particular, for completeness I describe both the "X+Y"
and "Y+X" configurations of each multiple level, when in some cases only one or
the other is commonly made into product. For example, I know that RAID 50 (5+0) is an
option in commercial RAID controllers, but I am not sure if anyone makes a RAID 05
solution. There may also be other combinations of RAID levels that I am not aware of.
Next: RAID X+Y vs. RAID Y+X
