Packard Bell 5800, ST8000 manual Glossary

B Glossary

	The following terms are used throughout this manual
	and may be helpful background information on the
	technology.
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 Flush	Refers to an operation where all un-written blocks in a Write-Back
	Cache are written to the target disk. This operation is necessary
	before powering down the system.
Channel	Refers to one of the SCSI bus connectors on the controllers or
	termination interface cards.
Consistency Check	Refers to a process where the integrity of redundant data is verified.
	For example, a consistency check of a mirrored drive will make sure
	that the data on both drives of the mirrored pair are exactly the same.
	For RAID level 3 and 5 redundancy, a consistency check will involve
	reading all associated data blocks, computing parity, reading parity,
	and verifying that the computed parity matches the read parity.
Disconnect/Reconnect	Disconnect is a function that allows a target SCSI device (typically a
	disk drive that received a request to perform a relatively long I/O
	operation) to release the SCSI bus so that the controller can send
	commands to other devices. When the operation is complete and the
	SCSI bus is needed by the disconnected target again, it is
	reconnected.
Disk Mirroring	Data written to one disk drive is simultaneously written to another disk
	drive. If one disk fails, the other disk can be used to run the system
	and reconstruct the failed disk.
Disk Spanning	Several disks appear as one large disk using this technology. This
	virtual disk can then store data across disks with ease without the
	user being concerned about which disk contains what data. The
	subsystem handles this for the user.
Disk Striping	Data is written across disks rather than on the same drive. Segment 1
	is written to drive 0, segment 2 is written to drive 1, and so forth until a
	segment has been written to the last drive in the chain. The next
	logical segment is then written to drive 0, then to drive 1, and so forth
	until the write operation is complete.
Duplexing	This refers to the use of two controllers to drive a disk subsystem.
	Should one of the controllers fail, the other is still available to provide
	disk I/O. In addition, depending how the controller software is written,
	both controllers may work together to read and write data
	simultaneously to different drives.
Fault-Tolerant	When something is fault-tolerant it is resistant to failure. A RAID 1
	mirrored subsystem, for example, is fault-tolerant because it can still
	provide disk I/O if one of the disk drives in a mirrored system fails.
Hot Spare	The “Hot Spare” is one of the most important features the controller
	provides to achieve automatic, non-stop service with a high degree of

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