Count Key Data
Encyclopedia
Count Key Data is a disk data architecture. Each physical disk record consists of a count field, an optional key field, and a ("user") data field with error correction/detection information appended to each field and gaps separating each field . Because of the gaps and other information the recorded space is larger than that required for just the count data, key data, or user data.
The principle behind the architecture is that since data record lengths can vary, they all have an associated count field which indicates the size of the key if used and the size of the data.. The count field has the identification of the physical location in cylinder-head-record format, the length of the key, and the length of the data. The key may be omitted or consist of a string of characters. Most often the key is omitted, the record located sequentially or by direct cylinder-head-record addressing. If it is present, the key is typically a copy of the first n bytes of the data record but can be any data which will be used to find the record. The key (and hence the record) is locatable via hardware commands. Since the introduction of IBM's System/360 in 1964 nearly all IBM
large and intermediate system DASD
s have used the Count Key Data Architecture. Compare to Fixed Block Architecture
(FBA).
The advantages of Count Key Data architecture are:
Reduced CPU and memory prices and higher device and interface speeds have somewhat nullified the advantages of CKD, and it is retained only because IBM's flagship operating system z/OS
does not support sector-oriented interfaces.
Extended Count Key Data (ECKD) refers to the CCW commands used with cached controllers for IBM DASD
. The new commands were introduced on the cached versions of the IBM 3880 and were extended on the IBM 3990. The ECKD channel commands provide improved performance for the obsolete Bus & Tag interface, ESCON
(or Enterprise Systems Connection) interface or the newer FICON
(Fiber Connectivity) protocol. ECKD allows the programmer to provide the control unit with information on intent and to perform operations in a single channel program that would require multiple channel programms with CKD.
The principle behind the architecture is that since data record lengths can vary, they all have an associated count field which indicates the size of the key if used and the size of the data.. The count field has the identification of the physical location in cylinder-head-record format, the length of the key, and the length of the data. The key may be omitted or consist of a string of characters. Most often the key is omitted, the record located sequentially or by direct cylinder-head-record addressing. If it is present, the key is typically a copy of the first n bytes of the data record but can be any data which will be used to find the record. The key (and hence the record) is locatable via hardware commands. Since the introduction of IBM's System/360 in 1964 nearly all IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
large and intermediate system DASD
Direct access storage device
In mainframe computers and some minicomputers, a direct access storage device, or DASD , is any secondary storage device which has relatively low access time relative to its capacity....
s have used the Count Key Data Architecture. Compare to Fixed Block Architecture
Fixed Block Architecture
Fixed Block Architecture is a disk layout in which each addressable record on disk is of the same size. The term fell out of use, since nearly all modern disk drives use this principle, termed logical block addressing and usually having a constant addressable block size of 512 bytes.- Count Key...
(FBA).
The advantages of Count Key Data architecture are:
- The record size can be exactly matched to the application block size
- CPU and memory requirements can be reduced by exploiting search-key commands.
- IBM CKD devices operate synchronously with the system channel and can process information in the gaps between the various fields, thereby achieving higher performance by avoiding the redundant transfer of information to the host.
Reduced CPU and memory prices and higher device and interface speeds have somewhat nullified the advantages of CKD, and it is retained only because IBM's flagship operating system z/OS
Z/OS
z/OS is a 64-bit operating system for mainframe computers, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions.Starting with earliest:*OS/VS2 Release 2 through Release 3.8...
does not support sector-oriented interfaces.
Extended Count Key Data (ECKD) refers to the CCW commands used with cached controllers for IBM DASD
Direct access storage device
In mainframe computers and some minicomputers, a direct access storage device, or DASD , is any secondary storage device which has relatively low access time relative to its capacity....
. The new commands were introduced on the cached versions of the IBM 3880 and were extended on the IBM 3990. The ECKD channel commands provide improved performance for the obsolete Bus & Tag interface, ESCON
ESCON
ESCON is a data connection created by IBM, and is commonly used to connect their mainframe computers to peripheral devices such as disk storage and tape drives. ESCON is an optical fiber, half-duplex, serial interface. It originally operated at a rate of 10 Mbyte/s, which was later increased to...
(or Enterprise Systems Connection) interface or the newer FICON
FICON
FICON is the IBM proprietary name for the ANSI FC-SB-3 Single-Byte Command Code Sets-3 Mapping Protocol for Fibre Channel protocol. It is a FC layer 4 protocol used to map both IBM’s antecedent channel-to-control-unit cabling infrastructure and protocol onto standard FC services and infrastructure...
(Fiber Connectivity) protocol. ECKD allows the programmer to provide the control unit with information on intent and to perform operations in a single channel program that would require multiple channel programms with CKD.