Glossary

Specifies that the controller begins using Read-Ahead if the two most recent disk accesses occurred in sequential sectors. This is the default read policy of the HP RAID 4Si controller.

Cache memory supports write-through or write-back caching, which can be selected for each logical drive. To improve performance in sequential disk accesses, the HP RAID 4Si controller uses write-through caching by default. You can disable write-through caching. With the battery pack, the cache memory provides up to 72 hours of data protection in the event of a power failure.

See stripe.

Dual Inline Memory Module. A small circuit board containing memory chips that fits into a 64-bit data bus (168-pin receptacle). Unlike a SIMM (Single Inline Memory Module), which fits into a 32-bit receptacle (72 pins) and must be used in pairs, DIMMs can be used one-at-a-time.

Either "error correction (or correcting) code" or "error checking and correcting." Allows data that is being read or transmitted to be checked for errors and when necessary, corrected "on the fly." It differs from parity-checking in that errors are not only detected but also corrected. ECC is increasingly being designed into data storage and transmission hardware as data rates (and therefore error rates) increase.

An HP RAID 4Si physical disk in a standby state, ready to begin rebuilding a failed physical drive.

The replacement of a hard drive, CD-ROM drive, power supply, or other device with a similar device while the computer system using it remains in operation. The replacement can be because of a device failure, or for storage devices, to substitute other data.

High Voltage Differential. A method of driving SCSI cables, which is being replaced by LVD.

Just a Bunch Of Disks. The group of physical disks before being assembled into a RAID.

A collection of physical disks combined into a RAID array and viewed by the user as one large drive.

On HP-UX, this term is most often used when referring to disk groups created by the Logical Volume Manager (LVM) software. Because logical RAID drives appear to the system as large SCSI disks, logical volumes might span logical drives. This can be useful, for example, to mirror data across multiple HP RAID 4Si controllers.

Low Voltage Differential. A method of driving SCSI cables. LVD uses less power than High voltage Differential (HVD), is less expensive, and allows the higher speeds of Ultra2 SCSI. LVD requires 3.3 Volts DC instead of the 5 Volts DC needed for HVD.

Mean Time Between Failures. A measure of how reliable a hardware product or component is. For most components, the measure is typically in thousands or even tens of thousands of hours between failures. For example, a hard disk drive might have an MTBF of 300,000 hours. A desired MTBF can be used as a quantifiable objective when designing a new product. The MTBF figure can be developed as the result of intensive testing, based on actual product experience, or predicted by analyzing known factors. The manufacturer might provide it as an index of a product's or component's reliability, and in some cases, to give customers an idea of how much service to plan for.

A "normal" caching policy specifies that the controller does not use Read-Ahead for the current logical drive.

An extra bit added to a byte or word to reveal errors in storage (in RAM or disk) or transmission. Parity is used to generate a set of redundancy data from two or more parent data sets. The redundancy data can be used to reconstruct one of the parent data sets. However, parity data does not fully duplicate the parent data sets. In RAID, this method is applied to entire drives or stripes across all disk drives in an array. Parity consists of dedicated parity, in which the parity of the data on two or more drives is stored on an additional drive, and distributed parity, in which the parity data is distributed among all the drives in the system. If a single drive fails, it can be rebuilt from the parity of the respective data on the remaining drives.

Peripheral Component Interface. The bus type used by the HP RAID 4Si controller to communicate with the HP-UX server.

Power Distribution Unit.

Redundant Array of Independent Disks. A group of physical disks, under the control of array management software, that work together to improve performance and decrease the odds of losing data due to mechanical or electronic failure.

Random Access Memory.

A memory caching capability in some controllers that allows them to read sequentially ahead of requested data and store the additional data in cache memory, anticipating that the additional data will be needed soon. Read-ahead supplies sequential data faster, but is not as effective when accessing random data.

See adaptive read-ahead.

Small Computer System Interface. An interface which acts like an expansion bus and allows the connection of peripherals such as hard disks, scanners, or CD-ROM drives in a chain (daisy-chaining). Depending on the SCSI level, you can chain up to 7 or 15 devices to a single SCSI channel.

Synchronous Dynamic RAM. A high-speed RAM that can synchronize itself with the clock speed of the microprocessor's data bus.

Single Inline Memory Module. See DIMM.

Data striping. Segmentation of logically sequential data, such as a single file, so that segments can be written to multiple physical devices (usually disk drives) in a round-robin fashion. This technique is useful if the processor is capable of reading or writing data faster than a single disk can supply or accept it. While data is being transferred from the first disk, the second disk can locate the next segment.

A type of cache memory scheme that stores data written to disk as well as data read from disk.

A cache memory scheme in which disk read operations are cached, but not disk write operations.