Virtual Count Key Data
Original Publication Date: 1993-Mar-01
Included in the Prior Art Database: 2005-Mar-18
The objective of Virtual Count Key Data (VCKD) is to create a count key data (CKD) appearance for a direct-access storage device (DASD) which may actually be a fixed block device.
Virtual Count Key Data
of Virtual Count Key Data (VCKD) is to create a
count key data (CKD) appearance for a direct-access storage device
(DASD) which may actually be a fixed block device.
home addresses and count fields for all tracks of
a cylinder in a separate fixed block area, called cylinder index.
The cylinder index portion, specifying the format of a single track,
is called track index. The key fields pertaining to a track are
packed in a separate fixed block area. Data fields are recorded such
that each data field occupies one fixed block or a sequence of blocks
as a single entity. This means, no other field is recorded in the
fixed block sequence dedicated to a data field. As a CKD field may
have any length, the last block in the sequence may be partly filled.
Each count field in the cylinder index is accompanied by a pointer to
the associated data field. If a record contains a key field, a key
field pointer addition ally accompanies the count field. This way
the fixed block addressing mechanism is used to link a count field
with the corresponding key and data field. As a data field is stored
as a single entity in a con tiguous fixed block area, it does not
have to be passed through as an intermediate buffer. It can be
transferred directly between the DASD and the system memory by means
of a regular read or write operation.
device is composed of a large number of virtual
cylinders mapped into the space of a fixed block device. Each
virtual cylinder is divided into a cylinder index area and a specific
number of virtual tracks (Figs. 1 and 3).
index (Fig. 2) consists of a cylinder index header
and a set of track indices, one for each track on the cylinder. The
content of the cylinder index header is implementation-dependent. It
contains inter alia a list of track index offsets identifying the
start of each track index, a list of key area pointers identifying
the start locations of the packed key fields for each track, and a
list of the track format identifiers.
entry representing a user record contains the record
count field and pointers to the corresponding key and data fields.
may be recorded following the data fields.
boundary between the fixed block area carrying data fields and the
fixed block area carrying key fields varies with the number and size
of the data fields. If no key fields exist, the entire space
assigned to a virtual track can be filled with data fields.
calculates the key field pointer and the data field
pointer and appends the...