Records, blocks, and formats

 

Records, blocks, and record formats of the tapes are basic concepts to understand when you use tape files.

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, 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.

 

Parent topic:

Concepts

 

Related concepts

Record formats