ABCs of z/OS System Programming Volume 3 - IBM Redbooks

Logical records and blocks
To an application, a logical record is a unit of information (for example, a customer, an
account, a payroll employee, and so on). It is the smallest amount of data to be processed,
and it is comprised of fields that contain information recognized by the processing application.
Logical records, when located in DASD or tape, are grouped into physical records named
blocks (to save space in DASD because of the gaps). Each block of data on a DASD volume
has a distinct location and a unique address (block number, track, and cylinder), thus making
it possible to find any block without extensive sequential searching. Logical records can be
stored and retrieved either directly or sequentially.
DASD volumes are used for storing data and executable programs (including the operating
system itself), and for temporary working storage. One DASD volume can be used for many
separate data sets, and space on it can be reallocated and reused. The maximum length of a
logical record (LRECL) is limited by the physical size of the media used.
Record format (RECFM)
RECFM specifies the characteristics of the logical records in the data set. They can have a:
► Fixed length (RECFM=F) - Every record is the same size.
► Variable length (RECFM=V) - The logical records can be of varying sizes; every record
has a preceding record descriptor word (RDW) to describe the length of such record.
► ASCII variable length (RECFM=D) - Used for ISO/ANSI tape data sets.
► Undefined length (RECFM=U) - Permits processing of records that do not conform to the
F or V format.
F, V, or D-format logical records can be blocked (RECFM=FB, VB or DB), which means
several logical records are in the same block.
Spanned records are specified as VS, VBS, DS, or DBS. A spanned record is a logical record
that spans two or more blocks. Spanned records can be necessary if the logical record size is
larger than the maximum allowed block size.
You can also specify the records as fixed-length standard by using FS or FBS, meaning that
there is not an internal short block.
Logical record length (LRECL)
LRECL specifies the length, in bytes, of each record in the data set. If the records are of
variable length or undefined length, LRECL specifies the maximum record length. For input,
the field has no effect for undefined-length (format-U) records.
Block size (BLKSIZE)
BLKSIZE specifies the maximum length, in bytes, of the physical record (block). If the logical
records are format-F, the block size must be an integral multiple of the record length. If the
records are format-V, you must specify the maximum block size. If format-V records are
unblocked, the block size must be 4 bytes (to specify the block length) greater than the record
length (LRECL). For data sets in DASD the maximum block size is 32760. For data sets in
tapes the maximum block size is much larger.
In an extended-format data set, the system adds a 32-byte suffix to each block, which is
transparent to the application program.
40 ABCs of z/OS System Programming Volume 3