IBM DS8000 User Manual page 219

data field
The optional third field of a count key
data (CKD) record. The count field
specifies the length of the data field. The
data field contains data that the program
writes.
data record
The basic unit of System z storage on a
DS8000, also known as a count-key-data
(CKD) record. Data records are stored on
a track. The records are sequentially
numbered starting with 0. The first
record, R0, is typically called the track
descriptor record and contains data that
the operating system normally uses to
manage the track. See also count-key-data
and fixed-block architecture.
data set FlashCopy
For System z hosts, a feature of
FlashCopy that indicates how many
partial volume FlashCopy relationships
are active on a volume.
data sharing
The ability of multiple host systems to
concurrently utilize data that they store
on one or more storage devices. The
storage unit enables configured storage to
be accessible to any, or all, attached host
systems. To use this capability, the host
program must be designed to support
data that it is sharing.
DDM See disk drive module.
DDM group
See disk pack.
dedicated storage
Storage within a storage unit 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 storage server demotes a data
unit to make room for other logical data
units in the cache or because the logical
data unit is not valid. The storage unit
must destage logical data units with
active write units before they can be
demoted. See also destage.
destage
To move data from an online or higher
priority to an offline or lower priority
device. The storage unit stages incoming
data into cache and then destages it to
disk.
device For System z, a disk drive.
device adapter (DA)
A physical component of the DS8000 that
provides communication between the
clusters and the storage devices. The
DS8000 has eight device adapters that it
deploys in pairs, one from each cluster.
Device adapter pairing enables the
DS8000 to access any disk drive from
either of two paths, providing fault
tolerance and enhanced availability.
device address
For System z, the field of an ESCON
device-level frame that selects a specific
device on a control-unit image.
device ID
The unique two-digit hexadecimal
number that identifies the logical device.
device interface card
A physical subunit of a storage cluster
that provides the communication with the
attached device drive modules.
device number
For System z, 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 device drive
module to a spare device drive module.
The subsystem maintains data access
during the process.
™
DFS See distributed file service.
DFSMS
See Data Facility Storage Management
Subsystem.
direct access storage device (DASD)
1) A mass storage medium on which a
computer stores data.
2) A disk device.
disk cage
A container for disk drives. Each disk
cage supports eight disk packs (64 disks).
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Glossary