Specifications

Coordinated Universal Time (UTC). The international
standard of time that is kept by atomic clocks around
the world.
Copy Services client. Software that runs on each
ESS cluster in the Copy Services server group and that
performs the following functions:
v Communicates configuration, status, and connectivity
information to the Copy Services server.
v Performs data-copy functions on behalf of the Copy
Services server.
Copy Services server group. A collection of
user-designated ESS clusters participating in Copy
Services functions managed by a designated active
Copy Services server. A Copy Services server group is
also called a Copy Services domain.
count field. The first field of a count key data (CKD)
record. This eight-byte field contains a four-byte track
address (CCHH). It defines the cylinder and head that
are associated with the track, and a one-byte record
number (R) that identifies the record on the track. It
defines a one-byte key length that specifies the length
of the record’s key field (0 means no key field). It
defines a two-byte data length that specifies the length
of the record’s data field (0 means no data field). Only
the end-of-file record has a data length of zero.
count key data (CKD). In Enterprise Systems
Architecture/390, a data-record format employing
self-defining record formats in which each record is
represented by up to three fields—a count area
identifying the record and specifying its format, an
optional key area that can be used to identify the data
area contents; and an optional data area that typically
would contain the user data for the record. For CKD
records on the ESS, the logical volume size is defined
in terms of the device emulation mode (3390 or 3380
track format). The count field is always 8 bytes long and
contains the lengths of the key and data fields, the key
field has a length of 0 to 255 bytes, and the data field
has a length of 0 to 65 535 or the maximum that will fit
on the track. Typically, customer data appears in the
data field. The use of the key field is dependent on the
software managing the storage. See data record.
CPC. See cluster processor complex.
CRC. See cyclic redundancy check.
CU. See control unit.
CUIR. See control-unit initiated reconfiguration.
customer console. See consoleand IBM TotalStorage
ESS Master Console.
CUT. See Coordinated Universal Time.
cyclic redundancy check (CRC). A redundancy
check in which the check key is generated by a cyclic
algorithm. (T)
cylinder. A unit of storage on a CKD device. A cylinder
has a fixed number of tracks.
D
DA. See device adapter and SSA adapter.
daisy chain. See serial connection.
DASD. See direct access storage device.
DASD fast write (DFW). Caching of active write data
by a storage server by journaling the data in nonvolatile
storage, avoiding exposure to data loss.
data availability. The degree to which data is
available when needed, typically measured as a
percentage of time that the system would be capable of
responding to any data request (for example, 99.999%
available).
data compression. A technique or algorithm used to
encode data such that the encoded result can be stored
in less space than the original data. The original data
can be recovered from the encoded result through a
reverse technique or reverse algorithm. See
compression.
Data Facility Storage Management Subsystem. An
operating environment that helps automate and
centralize the management of storage. To manage
storage, DFSMS provides the storage administrator with
control over data class, storage class, management
class, storage group, and automatic class selection
routine definitions.
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 S/390 and zSeries
storage on an ESS, 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 normally used by the
operating system to manage the track. See
count-key-data and fixed-block architecture.
data sharing. The ability of homogeneous or divergent
host systems to concurrently utilize data that they store
on one or more storage devices. The storage facility
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 drive module group.
188 ESS Host Systems Attachment Guide
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