Specifications

dedicated storage. Storage within a storage facility
that is configured such that a single host system has
exclusive access to the storage.
demote. To remove a logical data unit from cache
memory. A subsystem demotes a data unit in order to
make room for other logical data units in the cache. It
might also demote a data unit because the logical data
unit is not valid. A subsystem must destage logical data
units with active write units before they can be demoted.
destaging. (1) Movement of data from an online or
higher priority to an offline or lower (2) priority device.
The ESS stages incoming data into cache and then
destages it to disk.
device. In Enterprise Systems Architecture/390, a disk
drive.
device adapter (DA). A physical component of the
ESS that provides communication between the clusters
and the storage devices. The ESS has eight device
adapters that it deploys in pairs, one from each cluster.
DA pairing enables the ESS to access any disk drive
from either of two paths, providing fault tolerance and
enhanced availability.
device address. In Enterprise Systems
Architecture/390, the field of an ESCON or FICON
device-level frame that selects a specific device on a
control-unit image.
device interface card. A physical subunit of a storage
cluster that provides the communication with the
attached DDMs.
device number. In Enterprise Systems
Architecture/390, a four-hexadecimal-character identifier,
for example 13A0, that the systems administrator
associates with a device to facilitate communication
between the program and the host operator. The device
number is associated with a subchannel.
device sparing. A subsystem function that
automatically copies data from a failing DDM to a spare
DDM. The subsystem maintains data access during the
process.
direct access storage device (DASD). (1) A mass
storage medium on which a computer stores data. (2) A
disk device.
disk drive. Standard term for a disk-based nonvolatile
storage medium. The ESS uses hard disk drives as the
primary nonvolatile storage media to store host data.
disk drive module (DDM). A field replaceable unit that
consists of a single disk drive and its associated
packaging.
disk drive module group. In the ESS, a group of
eight disk drive modules (DDMs) contained in an 8-pack
and installed as a unit.
disk group. In the ESS, a collection of seven or eight
disk drives in the same SSA loop and set up by the
ESS to be available to be assigned as a RAID-5 rank.
You can format a disk group as CKD or FB, and as
RAID or non-RAID, or leave it unassigned.
DNS. See domain name system.
domain. (1) That part of a computer network in which
the data processing resources are under common
control. (2) In TCP/IP, the naming system used in
hierarchical networks. (3) A Copy Services server group,
in other words, the set of clusters designated by the
user to be managed by a particular Copy Services
server.
domain name system (DNS). In TCP/IP, the server
program that supplies name-to-address translation by
mapping domain names to internet addresses. The
address of a DNS server is the internet address of the
server that hosts the DNS software for the network.
drawer. A unit that contains multiple DDMs and
provides power, cooling, and related interconnection
logic to make the DDMs accessible to attached host
systems.
drive. (1) A peripheral device, especially one that has
addressed storage media. See disk drive module. (2)
The mechanism used to seek, read, and write
information on a storage medium.
duplex. (1) Regarding ESS Copy Services, the state
of a volume pair after PPRC has completed the copy
operation and the volume pair is synchronized. (2) In
general, pertaining to a communication mode in which
data can be sent and received at the same time.
dynamic sparing. The ability of a storage server to
move data from a failing disk drive module (DDM) to a
spare DDM while maintaining storage functions.
E
E10. The forerunner of the F10 model of the ESS. See
F10.
E20. The forerunner of the F20 model of the ESS. See
F20
EBCDIC. See extended binary-coded decimal
interchange code.
EC. See engineering change.
ECKD. See extended count key data.
electrostatic discharge (ESD). An undesirable
discharge of static electricity that can damage
equipment and degrade electrical circuitry.
emergency power off (EPO). A means of turning off
power during an emergency, usually a switch.
Glossary 189
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