"Fast, cheap, good: choose two"
The phrase above is a famous one in the world of computing, and elsewhere: it's a good
rule of thumb to keep in mind when making many purchase decisions. It describes well what
I call the "essential tradeoff triangle" that applies to thousands of different
technologies and devices, from PCs to race car engines to kitchen blenders: you can easily
get something that is inexpensive and fast, but at the cost of quality; or something high
quality and fast, but it won't be cheap; or something cheap and high quality, but of lower
performance. In RAID, the "good" attribute refers specifically to reliability concerns, and in particular, fault tolerance. The "fast" attribute refers
to either performance or capacity (it's not speed, but it does trade off against fault
tolerance and cost). In general, for a given price point, the performance improvement of a
RAID array trades off to some extent with the array's redundancy and data security
potential. If you want to improve both, you have to pay more. The same applies to
capacity: high capacity trades off against economy and fault tolerance. (These are of
course rules of thumb: the attributes don't strictly trade off against one
another in a linear way; I'm just talking about general trends.)
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This triangle shows how performance (or capacity), cost
and fault tolerance
trade off in the RAID world. At each of the corners, one of the attributes
is maximized at the expense of the other two. Point "A" represents a balance
between the three: not great in any of them, not poor in any either. Points "B",
"C" and "D" represent doing well in two of the attributes at the
expense of the
third: these are the "choose two" points that were mentioned earlier. |
What it all really comes down to is: what are your priorities, and what are you willing
to spend? If high data availability and fault tolerance are not important, you can get a
high-performance RAID array for relatively little money; if peak performance is not all
that important, you can get a very reliable system without spending a fortune. It's only
when you need both that things get expensive. The tradeoffs between performance, cost and
reliability are most readily seen when contrasting the different RAID levels. Some emphasize performance over fault tolerance,
others do the opposite, while some try to balance the attributes. Some cost more than
others. Some offer very high fault tolerance but relatively low capacity. In no case,
however, is there a "free lunch". Cheap RAID solutions are limited in their
ability to protect your data or improve performance; high-end RAID implementations that
provide very high performance and very high data reliability are also quite expensive.
Next: RAID Limitations
