Free blocks

Free blocks do not contain data. However, they have already been used at least once by ISAM and continue to be managed by ISAM. They result from block splitting operations or from deletion of records from the file.

Free blocks must not be confused with “unused blocks”: the free blocks belong to the “area in use” of an ISAM file and the space they occupy cannot be released by means of FILE (operand SPACE) or MODIFY-FILE-ATTRIBUTES; “unused blocks” form the unused part of the ISAM file and the space they occupy can be released at any time.

If new data, index or overflow blocks are needed, any existing free blocks are used before the unused blocks are used.