Mylex AcceleRAID 250 Informations techniques télécharger pdf (Page 223)

Glossary

Manual No. 775029 G-7

Disk

A non-volatile, randomly addressable, re-writable data storage device,

including rotating magnetic and optical disks as well as solid-state disks or

other electronic storage elements.

Disk Array

A collection of disks from one or more accessible disk systems combined

with an array management software. The array management software

controls the operation of the disks and presents them as one or more system

drives (virtual disks) to host operation environments. Disk arrays are

commonly used on severs and are becoming more popular on desktops and

workstations. An example of a disk array is the Mylex AcceleRAID family

or eXtremeRAID.

Disk Drive

A device for the electronic digital storage of information.

Disk Failure Detection

A RAID controller automatically detects SCSI disk failures. A monitoring

process running on the controller checks, among other things, elapsed time

on all commands issued to disks. A time-out causes the disk to be “reset”

and the command to be retried. If the command times out again, the disk

could be taken “offline” by the controller (its state changed to “dead”).

Mylex DAC960 controllers also monitor SCSI bus parity errors and other

potential problems. Any disk with too many errors will also be taken offline.

Disk Media Error Management

Mylex DAC960 controllers transparently manage SCSI disk media errors.

Disks are programmed to report errors, even ECC-recoverable errors. If ECC

EDO RAM is installed, the controller will correct ECC errors. When a disk

reports a media error during a read, the controller reads the data from the

mirror (RAID 1 or 0+1), or computes the data from the other blocks (RAID

3, RAID 5), and writes the data back to the disk that encountered the error. If

the write fails (media error on write), the controller issues a “reassign”

command to the disk, and then writes the data to a new location. Since the

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Mylex AcceleRAID 250 Informations techniques Page 223