Count key data
Count key data (CKD) also refers to the set of channel commands (collectively Channel Command Words, CCWs) that are generated by an IBM mainframe for execution by a DASD subsystem employing the CKD recording format.
[6][page needed] The advantages of count key data record format 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.
Initially there was a high degree of correspondence between the logical view of DASD accesses and the actual hardware, as shown in the illustration.
Three digit labels were typically affixed[d] to identify the address of channel, control unit and device.
On low end systems the Channel and the Control Unit were frequently physically integrated but remained logically separate.
[g] Access to specific classes of I/O devices by an IBM mainframe is under the control of Channel Command Words (CCWs), some of which are generic (e.g. No Operation) but many of which are specific to the type of I/O device (e.g. Read Backwards for a tape drive).
The initial feature set provided by IBM with its 1964 introduction of the CKD track format and associated CCWs included: .
Subsequently, the feature set was implemented on the 2314 family of storage controls and an integrated attachment of the System 370 Model 25.
The channel and storage control under certain conditions can inter-operate to cause a CCW to be retried without an I/O interruption.
Rotational position sensing (RPS) was implemented with two new CCWs, SET SECTOR and READ SECTOR enabled the channel to delay command chaining until the disk rotated to a specified angular track position.
The control unit implements RPS by dividing each track into equal angular segments.
Defect skipping was essentially a factory only feature until 1981 when CCWs for management along with associated utilities were released.
[24] Three additional commands, Set Path Group ID, Sense Path Group ID, and Suspend Multipath Reconnection, are used to support attachment of the 3380 Models having two controllers at the head of a string.
In addition, on 370-XA systems which set the multipath mode bit in the function control byte (byte 0) to a 1, block multiplex reconnections will occur on the first available path which is a member of the group over which the channel program was initiated (regardless of the reservation state of the device).
Nonsynchronous operation provided by the Extended CKD ("ECKD") set of CCWs removed the gap timing constraint.
An intermediate buffer in the storage control allows independent operations between the channel and the device.
A major advantage of ECKDs is far longer cables; depending upon application it may improve performance.
For example: Reduced CPU and memory prices and higher device and interface speeds have somewhat nullified the advantages of CKD, and support continues by IBM to this date because its flagship operating system z/OS continues to use CKD CCWs for many functions.
To bridge between the native fixed block sized disks and the variable length ECKD/CKD record format, the CKD track images in memory are mapped onto a series of fixed blocks suitable for transfer to and from an FBA disk subsystem.
