Track Format

Information is recorded on all DASD volumes in a standard format. This format is called count-key data (CKD) or extended count-key data (ECKD™).

Each track contains a record 0 (also called track descriptor record or capacity record) and data records. Historically, S/390® hardware manuals and software manuals have used inconsistent terminology to refer to units of data written on DASD volumes. Hardware manuals call them records. Software manuals call them blocks and use “record” for something else. The DASD sections of this document use both terms as appropriate. Software records are described in Organizing VSAM Data Sets and Selecting Record Formats for Non-VSAM Data Sets.

For these data formats, one or more of the following is true:

Each VSAM control interval consists of one or more contiguous blocks. Control intervals are grouped into control areas.
Each non-VSAM block contains part of a record or one or more records. Examples of these programming interfaces are BSAM, BDAM, and EXCP.
Each VSAM record occupies multiple control intervals or all or part of a control interval. Each non-VSAM record occupies multiple blocks or all or part of a block. An example is QSAM.
The application program might regard byte streams as being grouped in records. The program does not see blocks. Examples of such programs include UNIX files and OAM objects.

The process of grouping records into blocks is called blocking. The extraction of records from blocks is called unblocking. Blocking or unblocking might be done by the application program or the operating system. In z/OS UNIX, blocking means suspension of program execution.

Under certain conditions, BDAM uses the data area of record zero to contain information about the number of empty bytes following the last user record on the track. This is called the track descriptor record.

Figure 1 shows the two different data formats, count-data and count-key-data, only one of which can be used for a particular data set. An exception is PDSs that are not PDSEs. The directory blocks are in count-key-data format, and the member blocks normally are in count-data format.

Figure 1. DASD Volume Track Formats

Count-Data Format: Records are formatted without keys. The key length is 0. The count area contains 8-bytes that identify the location of the block by cylinder, head, and record numbers, and its data length.

Count-Key-Data Format: The blocks are written with hardware keys. The key area (1 - 255 bytes) contains a record key that specifies the data record, such as the part number, account number, sequence number, or some other identifier.

In data sets, only BDAM, BSAM, EXCP, and PDS directories use blocks with hardware keys. Outside data sets, the VTOC and the volume label contain hardware keys.

Tip: The use of hardware keys is less efficient than the use of software keys (which VSAM uses).