Spanned blocked and unblocked files
A spanned blocked QSAM file is made up of blocks, each containing one or more logical records or segments of logical records. A spanned unblocked file is made up of physical blocks, each containing one logical record or one segment of a logical record.
In a spanned blocked file, a logical record can be either fixed or variable in length, and its size can be smaller than, equal to, or larger than the physical block size. There are no required relationships between logical records and physical block sizes.
In a spanned unblocked file, the logical records can be either fixed or variable in length. When the physical block contains one logical record, the block length is determined by the logical record size. When a logical record has to be segmented, the system always writes the largest physical block possible. The system segments the logical record when the entire logical record cannot fit on a track.