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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 zSeries 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 zSeries 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 zSeries, 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 zSeries, 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 zSeries, 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.
Glossary 331