Ablative. Ablative technology utilizes heat to remove
a layer of some material. In this context, it relates to
writing information with a laser by using the lasers
heat to burn away a layer of the recording medium,
thereby representing a binary value.
Access pattern skew. This refers to the tendency for
reference to information to follow a pattern whereby,
information is referenced from a certain area for
some time, then another area, then a further one,
rather than completely random jumps. Thus, if
information in a file was written sequentially across a
number of disks, utilization would tend towards one
disk at a time. If however, the file data is split into
blocks, and each block written to a separate disk, the
skew is eliminated.
Access time. This refers to the total length of time
from the initiation of a request for data, to the start of
the receipt of that data from a device.
Actuators. An actuator is the mechanical assembly
that is responsible for moving the disk head back and
forth across the disk surface.
Allocation group. An allocation group consists of a
set of i-node pointers to data blocks, and the data
blocks themselves. This is a file system entity used to
improve access to files within a file system based on
locality of reference.
Areal density. This defines the density at which
individual bits can resolved by the read head. This
equates to the maximum bit density supported by the
Archive. Archiving involves moving data from the
location that it is usually accessed from (normally
fast, expensive storage), to lower cost storage such
as tape. Information is normally archived if access to
it will be very infrequent. Contrast with retrieval.
Asynchronous. This refers to an operation that can
occur independently of other operations. An
asynchronous communication for example, can be
sent and then other work initiated without waiting for
a response. Contrast with synchronous.
Autochanger. An autochanger is a mechanical device
designed to load an remove media from a drive
automatically. Tape and optical libraries have
Backup. Backup involves taking a copy of data,
usually on some form of removable media, so that in
the event that information is lost, it can be easily
recovered. Contrast with restore.
Bad block relocation. When a write of a block of data
to a disk occurs, some software (and in some cases
the hardware), is capable of detecting that the write
failed (usually with a read following the write to test).
In this case, transparently to the process that
requested the write, the hardware or software can
mark the block as bad so that it will not be used
again, and redirect the write to a fresh block.
Banding. Traditionally, writing of bits to a disk
surface occurs in a regular fashion; thus the further in
toward the center of the disk, the less information can
be stored. Banding refers to a process of dividing the
disk surface into a number of concentric regions. As
the disk write head moves into regions closer to the
center of the disk, the bit write frequency increases
proportionally, thereby maintaining the bit density.
Block. A block is a unit of data to be written or read.
There are various block sizes, depending upon the
media and software. Disk device drivers currently use
a block size of 512 bytes to write to the disk.
Bus. A bus is a data and control path between
devices. It consists of power lines, a number of data
lines, and a number of control lines. There are
various standards including Micro Channel and PCI.
Cache. A cache is a area of extremely fast (usually
expensive) memory that is used to maintain
frequently accessed information, or store information
temporarily. Caches are used in various parts of a
computer system. In disk subsystem controllers for
example, writes to disk will actually occur to the
cache so that a completion return code can be quickly
returned to the writing process. The actual write will
occur from the cache when the subsystem has time to
satisfy it. The CPU also maintains several caches
where instructions and data can be pre-loaded while
the current instruction is executing.
Caddy. A caddy is a removable casing that a
CD-ROM is placed in before being loaded into the
CCW. Continuous composite write describes the
magneto-optical implementation of WORM. Erasure
and rewriting are prevented by simply not allowing
the functions to take place. Contrast with WORM.
CD-ROM. A CD-ROM is an optical disk that has
information stored on it before it is distributed. The
information is permanently stored and cannot be
erased or rewritten.
Command tag queueing. Command tag queuing
refers to the SCSI-2 implementation of
commands together to a device on the SCSI bus.
Copyright IBM Corp. 1994 355