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DIAL RAID FOR INTEGRITY

In each of the previous examples, the fault tolerance built into the systems is generally designed to preserve the availability of the hardware system. RAID is a technology that is probably the most popular means of ensuring the integrity of corporate data. RAID (redundant arrays of independent disks) is a way of coordinating multiple disk drives to protect against loss of data availability if one of the drives fails. RAID software:

  Presents the array’s storage capacity to the host computer as one or more virtual disks with the desired balance of cost, data availability, and I/O performance.
  Masks the array’s internal complexity from the host computer by transparently mapping its available storage capacity onto its member disks and converting I/O requests directed to virtual disks into operations on member disks.
  Recovers data from disk and path failures and provides continuous I/O service to the host computer.

RAID technology is based on work that originated at the University of California at Berkeley in the late 1980s. Researchers analyzed various performance, throughput, and data protection aspects of the different arrangements of disk drives and different redundancy algorithms. The following table describes the various RAID levels recognized by the RAID Advisory Board (RAB), which sets standards for the industry.

Description Benefits Disadvantages
RAID 0 Disk stripping; data is written across all drives, multiple disk drives. Storage is maximized; tolerance features good performance and low price. Has virtually no fault.
RAID 1 Disk mirroring; data is copied from one drive to the next. Data redundancy is increased 100%; has fast read performance. Slower write performance, but twice the disk drive capacity; more expensive.
RAID 2 Spreads redundant data across multiple disks; includes bit and parity data checking. Has no physical benefits. Has high overhead with no significant reliability.
RAID 3 Data stripping at a bit level; requires a dedicated parity drive. Has increased fault tolerance and fast performance. Is limited to one write at a time.
RAID 4 Disk stripping of data blocks, requires a dedicated parity drive. Has increased fault tolerance and fast read performance. Slower write performance; not used very much.
RAID 5 Disk stripping of both data and parity information. Features increased fault tolerance, efficient performance, is very common. Write performance is slow.

The redundancy in RAID is achieved by dedicating parts of an array’s storage capacity to check data. Check data can be used to regenerate individual blocks of data from a failed disk as they are requested by the applications, or to reconstruct the entire contents of a failed disk to restore data protection after a failure.

The most common forms of check data are a mirror (identical copy) of user data and shared parity, which involves appending mathematical code to data bits for later comparison, matching, and correction. Different combinations of mapping and check data comprise distinct RAID levels.

Striping

Of the six well-defined RAID levels, three are commonly used. Level 1 uses mirroring for data protection and may incorporate striping. Striping refers to the location of consecutive sequences of data blocks on successive array members. Striping balances I/O load, thereby increasing performance.

Levels 3 and 5 both use parity for data protection and almost always incorporate striping. RAID levels 3 and 5 use different algorithms for updating both user data and check data in response to application write requests.

In a RAID level 3 array, the disks are physically or logically synchronized, and each contributes to satisfying every I/O request made to the array (i.e., parallel access). In a RAID level 5 array, the disks are allowed to operate independently (i.e., independent access) so that in principle, the array may satisfy multiple application I/O requests concurrently.

Some RAID levels are theoretically faster than others, but in many situations the existing hardware technology does not always enable these performance enhancements to be realized. Other factors that are significant in overall system performance include the combinations of the disk drive, the host adapter, the tuning of the operating system, and how these components function together.

Parity

In RAID level 3, parity information that is saved to one designated tape drive can be used to regenerate data from a failed drive or tape media (see Exhibit 1-7-1).


Exhibit 1-7-1.  Example of RAID 3

RAID 5 offers improved storage efficiency over RAID 1 because parity information is stored rather than a complete redundant copy of all data. The parity information is essentially a number determined by adding up the value of all the bits in the data word. Parity requires some amount of overhead, ranging from 50% on RAID 1 to somewhat less than 20% on RAID 5. The result is that three or more identical drives can be combined into a RAID 5 array, with the effective storage capacity of only one drive sacrificed to store the parity information. Therefore, RAID 5 arrays provide greater storage efficiency than RAID 1 arrays.

RAID 5 is an implementation in which parity information is striped across all the configured drives. This method can increase the array throughput by separating the parity information across all drives. This is the preferred method when using transaction or data base processing (see Exhibit 1-7-2).


Exhibit 1-7-2.  Example of RAID 5

Data blocks and parity blocks are striped to the drives or tapes in a stair-step, or barber-pole fashion, allowing for full restoration even if a disk or tape is lost or damaged. In such an event, the data and parity blocks on the remaining drives or tapes contain enough information for the software to extrapolate the “lost” data.


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