Count Key Data

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Count Key Data (CKD) is a disk data architecture. Each physical disk record consists of a count area, an optional key, and a data record followed by an inter-record gap. 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 physical record size.[1]. The count area has the identification of the physical location in cylinder-head-record format, the length of the key, and the length of the data record. 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. [2] The Count Key Data Architecture was created by IBM for its System/360 disk drives in the 1960s. Compare to Fixed Block Architecture (FBA).

The advantage of Count Key Data architecture is that the physical record size can be exactly matched to a multiple of the logical record size. The first advantage of "blocking" these records together is that multiple records can be handled in a single sequence of hardware operations saving CPU cycles, reducing I/O channel utilization, and average time to read or write data. The second advantage is that because there are no "unused" bytes in a physical record and the number of inter-record gaps is reduced, there is a savings in disk space.

Extended Count Key Data (ECKD) refers to the format used with IBM mainframe storage. Under IBM System Storage, the ECKD formatted channel command provide improved performance for the ESCON (or Enterprise Systems Connection) interface or the newer FICON (Fiber Connectivity) protocol. ECKD separates many of the physical aspects of DASD from the logical address in the channel program just as a cached control unit for disk separates the physical hardware from the logical. With ECKD, the physical track is the smallest unit. The records on the track, although formatted just as for CKD are written as a unit in a single operation.

[edit] References

  1. ^ Count key data, IBM infocenter
  2. ^ Houtekamer, Gilbert E. and Artis, H. Pat, MVS I/O Subsystems, McGraw-Hill 1991, ISBN 0-07-002553-3
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