Dell PERC 4/SI - Manual (Page 70)

Disk Array
A collection of disks from one or more disk subsystems controlled by array management software. The array management software controls the disks and
presents them to the array operating environment as a virtual disk.
Disk Duplexing
A variation on disk mirroring in which a second disk adapter or host adapter and redundant disk drives are present.
Disk Mirroring
Writing duplicate data to more than one (usually two) hard disks to protect against data loss in the event of device failure. Disk mirroring is a common feature
of RAID systems.
Disk Spanning
The process of creating one logical drive composed of multiple arrays. Spanning is used to create complex RAID sets, such as RAID levels 10 and 50. Spanning
utilizes striping to distribute data across all member disk drives.
Disk Striping
A type of disk array mapping. Consecutive stripes of data are mapped round-robin to consecutive array members. A striped array (RAID level 0) provides high
I/O performance at low cost, but provides less data reliability than any member disk.
Disk Subsystem
A collection of disks and the hardware that controls them and connects them to one or more controllers. The hardware can include an intelligent adapter, or
the disks can attach directly to a system I/O bus adapter.
Double Buffering
A technique that achieves maximum data transfer bandwidth by constantly keeping two I/O requests outstanding for adjacent data. A software component
begins a double-buffered I/O stream by issuing two requests in rapid sequence. Thereafter, each time an I/O request completes, another is immediately
issued. If the disk subsystem can process requests fast enough, double buffering allows data to be transferred at the full-volume transfer rate.
Drive Roaming
Drive roaming occurs when the hard drives are changed to different target IDs or channels on the same controller. (A single-channel adapter can perform drive
roaming.) When the drives are placed on different channels or target IDs, the controller detects the RAID configuration from the configuration information on
the drives. Configuration data is saved in both NVRAM on the RAID controller and in the hard drives attached to the controller. This maintains the integrity of
the data on each drive, even if the drives have changed their target ID.
Failed Drive
A drive that has ceased to function, that consistently functions improperly, or that is inaccessible.
Fast SCSI
A variant on the SCSI-2 bus. It uses the same 8-bit bus as the original SCSI-1 but runs at up to 10MBs (double the speed of SCSI-1).
Firmware
Software stored in read-only memory (ROM) or Programmable ROM (PROM). Firmware is often responsible for the behavior of a system when it is first turned
on. A typical example would be a monitor program in a system that loads the full operating system from disk or from a network and then passes control to the
operating system.
Format
The process of writing a specific value to all data fields on a physical drive, (hard drive), to map out unreadable or bad sectors. Because most hard drives are
formatted when manufactured, formatting is usually done only if a hard disk generates many media errors.
GB
A gigabyte; 1,000,000,000 (10 to the ninth power) bytes.
Host System
Any system to which disks are directly attached, (not attached remotely.) Mainframes, workstations, and personal computers can all be considered host
systems.
Hot Spare
An idle, powered on, stand-by drive ready for immediate use in case of disk failure. It does not contain any user data. Up to eight disk drives can be assigned
as hot spares for an adapter. A hot spare can be dedicated to a single redundant array or it can be part of the global hot-spare pool for all arrays controlled
by the adapter.
When a disk fails, the controllers' firmware automatically replaces and rebuilds the data from the failed drive to the hot spare. Data can be rebuilt only from
logical drives with redundancy (RAID levels 1, 5, 10, or 50; not RAID 0), and the hot spare must have sufficient capacity. The system administrator can replace
the failed disk drive and designate the replacement disk drive as a new hot spare.
Hot Swap
The manual replacement of a failed drive while the disk subsystem is running (performing its normal functions).