What is a RAID System?
The Berkeley papers do not provide a concise definition of the term RAID.
Instead, they propose RAID schemes as an inexpensive method for obtaining
significant increases in I/O bandwidth, and then provide an implied definition of RAID by
an example of the architectures. Unfortunately, not all RAID architectures improve
bandwidth. Therefore, we propose the following definition which incorporates the
models presented here and that also embodies the manner in which RAID is used in
A Redundant Array of Inexpensive Disks (RAID) is any disk subsystem architecture that combines two or more standard physical disk drives into a single logical drive in order to achieve data redundancy.
We are deliberately leaving some terms, such as “standard physical disk drive” vague,
since the meaning of such a term changes from year to year. Note that the definition
says nothing about improved performance. The only criteria for a disk array to be a
RAID is that it provides data redundancy. Performance can even degrade compared to a
single drive case.
In practice, many of the reasons that we would use a RAID architecture also require
improved performance, and the primary motivation for most of the differences among
the RAID architectures is improved performance in one area or another. But, if we
simply build an array in order to improve performance through parallel disk activity
and do not provide any data redundancy in the architecture, we do not have a true RAID
Real World Applicability
In the posts that follow, we will formally define and discuss the relative strengths
and weaknesses of five basic types of RAID architectures followed by expanded or “nested”
RAID schemes. It should be remembered that these implementations are described as they
are defined in the Berkeley papers and that they are not inclusive of all of the viable array
alternatives. Also, particular vendors may or may not have implemented their products in
strict accordance with the definitions. Therefore, simply because a vendor lists
their product as being a RAID 5 implementation, it should not automatically be expected
to exhibit all of the advantages or disadvantages of the RAID 5 definition. Rather, what it
means is that potential customers have heard about the RAID definitions and they want
to know which type of RAID the vendor has implemented. When the vendor says that it
has a RAID 5 implementation, what it really means is that RAID 5 is the RAID definition
that best describes the vendor product, not that it exactly describes the product.
With the above caution in mind, potential RAID purchasers should use this information
to understand the key implementation differences between architectures and their
relative strengths and weaknesses. Armed with this understanding, actual vendor
offerings may be evaluated against the user requirements to determine applicability.