Read this information to help you understand
records, blocks, and record formats of the tapes.
- Records
- Records are logical mappings of the data on a tape. It typically maps
directly to the records in a database file.
- Blocks
- Blocks are physical units of data on a tape. Blocks can include a record,
part of a record, or multiple records.
- Record block format
- The record block format allows the system and user to interpret the data
on a tape.
To understand records, blocks, and record block formats, you must know
a few key terms:
- Block Descriptor Word (BDW)
- One or more logical records or record segments follow a block descriptor
word (BDW) in a variable length block.
- Blocked records
- Blocking is the process of grouping records into blocks before the system
writes records on a volume. A block consists of one or more logical records.
Blocking conserves storage space on a volume by reducing the number of interblock
gaps in the data set. This increases processing efficiency by reducing the
number of I/O operations that are required to process the data set.
- Deblocked records
- One record exists per block.
- Fixed length
- The blocks on a tape have an exact (or fixed) length.
- Interblock gap
- Interblock gap is the physical gap on tape between two data blocks.
- Record Descriptor Word (RDW)
- Data follows a record descriptor word (RDW) in a variable length logical
record. The RDW describes the record.
- Record Segment
- As spanned records occupy more than one block, each part of the record
is a record segment.
- Segment Descriptor Word (SDW)
- Data follows a segment descriptor word (SDW) in each record segment. The
SDW, similar to the RDW, describes the record segment.
- Spanned records
- The system splits a single record (spans) into two different data blocks
and writes them on the tape.
- Undefined length
- The blocks on a tape have no defined length; each block can be different;
and the program application interprets each block correctly.
- Unspanned records
- Each record is contained within one data block.
- Variable length
- The blocks on a tape have a variable length. The block contains a header
with the length of the block. Each block in a file might or might
not have the same length.
In sorting out these terms, the program supports and translates certain
combinations into record block formats.