The standard RAID levels are a basic set of RAID configurations that employ the techniques
of striping, mirroring, or parity to create large reliable data stores from general purpose
computer hard disk drives. The most common types today are RAID 0, RAID 1 and variants,
RAID 5 and RAID 6. RAID levels and their associated data formats are standardized by
the Storage Networking Industry Association in the Common RAID Disk Drive Format standard.
RAID 0
A RAID 0 splits data evenly across two or more disks without parity information for
speed. RAID 0 was not one of the original RAID levels and provides no data redundancy.
RAID 0 is normally used to increase performance, although it can also be used as a way to create
a large logical disk out of two or more physical ones.
A RAID 0 can be created with disks of differing sizes, but the storage space added to the
array by each disk is limited to the size of the smallest disk. For example, if a 100 GB
disk is striped together with a 350 GB disk, the size of the array will be 200 GB.
The diagram shows how the data is distributed into Ax stripes to the disks. Accessing the
stripes in the order A1, A2, A3, ... provides the illusion of a larger and faster drive.
Once the stripe size is defined on creation it needs to be maintained at all times.
Performance RAID 0 is also used in areas where performance
is desired and data integrity is not very important, for example in some computer gaming
systems. Although some real-world tests with computer games showed a minimal performance
gain when using RAID 0, albeit with some desktop applications benefiting, another article
examined these claims and concluded: "Striping does not always increase performance, but
in most situations it will yield a significant improvement in performance."
RAID 1
An exact copy of a set of data on two disks. This is useful when read performance or reliability
is more important than data storage capacity. Such an array can only be as big as the smallest
member disk. A classic RAID 1 mirrored pair contains two disks.
RAID 2
A RAID 2 stripes data at the bit level, and uses a Hamming code for error correction.
The disks are synchronized by the controller to spin at the same angular orientation, so
it generally cannot service multiple requests simultaneously. Extremely high data transfer
rates are possible. This is the only original level of RAID that is not currently used.
All hard disks eventually implemented Hamming code error correction. This made RAID 2 error
correction redundant and unnecessarily complex. This level quickly became useless and is now
obsolete. There are no commercial applications of RAID 2.
RAID 3
A RAID 3 uses byte-level striping with a dedicated parity disk. RAID 3 is very rare
in practice. One of the characteristics of RAID 3 is that it generally cannot service
multiple requests simultaneously. This happens because any single block of data will, by
definition, be spread across all members of the set and will reside in the same location.
So, any I/O operation requires activity on every disk and usually requires synchronized
spindles. This makes it suitable for applications that
demand the highest transfer rates in long sequential reads and writes, for example uncompressed
video editing. Applications that make small reads and writes from random disk locations
will get the worst performance out of this level.
The requirement that all disks spin synchronously, a.k.a. lockstep, added design considerations
to a level that didn't give significant advantages over other RAID levels, so it quickly became
useless and is now obsolete. Both RAID 3 and RAID 4 were quickly replaced by RAID 5.
RAID 3 was usually implemented in hardware, and the performance issues were addressed
by using large disk caches.
RAID 4
A RAID 4 uses block-level striping with a dedicated parity disk.
In the example on the right, a read request for block A1 would be serviced by disk 0.
A simultaneous read request for block B1 would have to wait, but a read request for B2 could
be serviced concurrently by disk 1. RAID 4 is very uncommon, but one enterprise
level company that has previously used it is NetApp. The aforementioned performance
problems were solved with their proprietary Write Anywhere File Layout, an approach to
writing data to disk locations that minimizes the conventional parity RAID write penalty.
By storing system metadata in the same way application data is stored, WAFL is able to
write file system metadata blocks anywhere on the disk. This approach in turn allows
multiple writes to be "gathered" and scheduled to the same RAID stripe—eliminating the
traditional read-modify-write penalty prevalent in parity-based RAID schemes.
RAID 5
A RAID 5 comprises block-level striping with distributed parity. Unlike in RAID 4, parity
information is distributed among the drives. It requires that all drives but one be present
to operate. Upon failure of a single drive, subsequent reads can be calculated from the
distributed parity such that no data is lost. RAID 5 requires at least three disks.
RAID 6
RAID 6 extends RAID 5 by adding an additional parity block; thus it uses block-level striping
with two parity blocks distributed across all member disks.
Performance RAID 6 does not have a performance penalty
for read operations, but it does have a performance penalty on write operations because of the
overhead associated with parity calculations. Performance varies greatly depending on how
RAID 6 is implemented in the manufacturer's storage architecture – in software, firmware
or by using firmware and specialized ASICs for intensive parity calculations. It can
be as fast as a RAID-5 system with one fewer drive.
Implementation According to the Storage Networking Industry
Association, the definition of RAID 6 is: "Any form of RAID that can continue to execute
read and write requests to all of a RAID array's virtual disks in the presence of any two concurrent
disk failures. Several methods, including dual check data computations, orthogonal dual
parity check data and diagonal parity, have been used to implement RAID Level 6."
Computing parity Two different syndromes need to be computed
in order to allow the loss of any two drives. One of them, P can be the simple XOR of the
data across the stripes, as with RAID 5. A second, independent syndrome is more complicated
and requires the assistance of field theory. To deal with this, the Galois field is introduced
with , where for a suitable irreducible polynomial of degree . A chunk of data can be written
as in base 2 where each is either 0 or 1. This is chosen to correspond with the element
in the Galois field. Let correspond to the stripes of data across hard drives encoded
as field elements in this manner. If is some generator of the field and denotes addition
in the field while concatenation denotes multiplication, then and may be computed as follows:
For a computer scientist, a good way to think about this is that is a bitwise XOR operator
and is the action of a linear feedback shift register on a chunk of data. Thus, in the
formula above, the calculation of P is just the XOR of each stripe. This is because addition
in any characteristic two finite field reduces to the XOR operation. The computation of Q
is the XOR of a shifted version of each stripe. Mathematically, the generator is an element
of the field such that is different for each nonnegative satisfying .
If one data drive is lost, the data can be recomputed from P just like with RAID 5.
If two data drives are lost or a data drive and the drive containing P are lost, the data
can be recovered from P and Q or from just Q, respectively, using a more complex process.
Working out the details is extremely hard with field theory. Suppose that and are the
lost values with . Using the other values of , constants and may be found so that and
:
Multiplying both sides of the equation for by and adding to the former equation yields
and thus a solution for , which may be used to compute .
The computation of Q is CPU intensive compared to the simplicity of P. Thus, a RAID 6 implemented
in software will have a more significant effect on system performance, and a hardware solution
will be more complex. Comparison
The following table provides an overview of some considerations for standard RAID levels.
In each case: Array space efficiency is given as an expression
in terms of the number of drives, ; this expression designates a fractional value between zero
and one, representing the fraction of the sum of the drives' capacities that is available
for use. For example, if three drives are arranged in RAID 3, this gives an array space
efficiency of thus, if each drive in this example has a capacity of 250 GB, then the
array has a total capacity of 750 GB but the capacity that is usable for data storage
is only 500 GB. Array failure rate is given as an expression
in terms of the number of drives, , and the drive failure rate, and can be seen to be
a Bernoulli trial. For example, if each of three drives has a failure rate of 5% over
the next three years, and these drives are arranged in RAID 3, then this gives an array
failure rate over the next three years of:
Non-standard RAID levels and non-RAID drive architectures
Alternatives to the above designs include nested RAID levels, non-standard RAID levels,
and non-RAID drive architectures. Non-RAID drive architectures are referred to by similar
acronyms, notably SLED, Just a Bunch of Disks, SPAN/BIG, and MAID.
Notes
References
External links RAID Calculator for Standard RAID Levels and
Other RAID Tools IBM summary on RAID levels
RAID 5 parity explanation and checking tool Animations and details on RAID levels 0, 1,
and 5 The Open-E Blog. "How does RAID 5 work? The
Shortest and Easiest explanation ever!"