B
bit
The smallest unit of digital data, representing a 0
or a 1. Abbreviated “b”.
block
See “data block”.
byte
A unit of data that is 8 bits long. Often used for
alphanumeric characters. Abbreviated “B”.
C
cache
Reserved areas of memory that are used to
speed up instruction execution, data retrieval,
and data updating. In Nexsan storage units, a
memory unit in the RAID controller that
temporarily holds user data.
CHAP
Challenge-Handshake Authenticaton Protocol. A
method of authenticating any two devices that
wish to communicate securely over a network.
CHAP uses a “shared secret”, which is a plain
text key known by both devices, to authenticate
the connection.
community
The administrative relationship between the
SNMP agent and the SNMP manager.
Community provides a means by which an
SNMP agent can correlate an incoming request
to an SNMP manager. Through this correlation,
the SNMP agent can determine the level of
authorization provided to the incoming request.
D
daisy-chain
The attachment of hardware to a computing
system by connecting each component to
another similar component rather than directly to
the computing system that uses the component.
Only the last component in the chain directly
connects to the computing system. For example,
up to two Nexsan expansion units can be daisy-
chained to the back of one Nexsan main storage
unit.
DAS
See “direct-attached storage”.
DAT
The file format of text-based event logs
downloaded from Nexsan storage units and of
snapshot and replication license files uploaded to
Nexsan storage units.
data block
A sequence of data that is a specific number of
bits (b) or bytes (B) long. When data is written to
a disk drive, it is organized into data blocks so
that it can be stored and retrieved more easily.
This is not the same thing as data striping, as
sequential data blocks are all stored on the same
disk drive. Sometimes a data block can be “bad”;
this does not necessarily mean that the data is
bad, only that the specific part of the surface of a
storage disk is unable to be read from or written
to. Most disk drives automatically quarantine bad
blocks as soon as they are detected so that they
do no disrupt data writing or retrieval.
data striping
The technique of segmenting logically sequential
data, such as a file, in a way that sequential
segments are stored on and retrieved from
different physical storage devices. Each
segment is called a “stripe”. Stripes on different
drives can be accessed at the same time,
increasing throughput and reducing access
delays. See also “stripe size” and “stripe width”.
Daytime
Short for “Daytime Protocol”. One of two
protocols for synchronizing a Nexsan storage
system’s internal clock with that of a time server.
dedicated spare
See “spare disk”.
Glossary
Nexsan High-Density Storage
User Guide
255