Glossary

Array

A grouping or array of disk drives combines the storage space on the disk drives into a single

 

segment of contiguous storage space. MegaRAID can group disk drives on one or more SCSI

 

channels into an array. A hot spare drive does not participate in an array.

Array Management Software Software that provides common control and management for a disk array. Array Management Software most often executes in a disk controller or intelligent host bus adapter, but can also execute in a host computer. When it executes in a disk controller or adapter, Array Management Software is often called firmware.

Array Spanning Array spanning by a logical drive combines storage space in two arrays of disk drives into a single, contiguous storage space in a logical drive. MegaRAID logical drives can span consecutively numbered arrays that each consist of the same number of disk drives. Array spanning promotes RAID levels 1, 3, and 5 to RAID levels 10, 30, and 50, respectively. See also Disk Spanning.

Asynchronous Operations Operations that bear no relationship to each other in time and can overlap. The concept of asynchronous I/O operations is central to independent access arrays in throughput-intensive applications.

Cache I/O

A small amount of fast memory that holds recently accessed data. Caching speeds subsequent

 

access to the same data. It is most often applied to processor-memory access, but can also be used

 

to store a copy of data accessible over a network. When data is read from or written to main

 

memory, a copy is also saved in cache memory with the associated main memory address. The

 

cache memory software monitors the addresses of subsequent reads to see if the required data is

 

already stored in cache memory. If it is already in cache memory (a cache hit), it is read from

 

cache memory immediately and the main memory read is aborted (or not started.) If the data is not

 

cached (a cache miss), it is fetched from main memory and saved in cache memory.

Channel

An electrical path for the transfer of data and control information between a disk and a disk

 

controller.

Consistency Check An examination of the disk system to determine whether all conditions are valid for the specified configuration (such as parity.)

Cold Swap A cold swap requires that you turn the power off before replacing a defective hard drive in a disk subsystem.

Data Transfer Capacity The amount of data per unit time moved through a channel. For disk I/O, bandwidth is expressed in megabytes per second (MB/s).

Degraded A drive that has become non-functional or has decreased in performance.

Disk A non-volatile, randomly addressable, rewritable mass storage device, including both rotating magnetic and optical disks and solid-state disks, or non-volatile electronic storage elements. It does not include specialized devices such as write-once-read-many (WORM) optical disks, nor does it include so-called RAM disks implemented using software to control a dedicated portion of a host computer volatile random access memory.

Cont’d

Glossary129