operating-systems file-management
Definition
Blocking
Blocking is the mapping of logical user records onto physical disk blocks. Since user records rarely match the fixed size of hardware sectors, the operating system must pack or split records to optimise storage utilisation.
Methods
Fixed Blocking
Records have fixed length, and an integral number of records fill each block.
Cost: Internal fragmentation — unused space at block end is wasted.
Variable Blocking — Spanned
Records have variable length and may span multiple block boundaries.
Benefit: Zero internal fragmentation.
Cost: Reading a spanned record requires at least two I/O operations.
Variable Blocking — Unspanned
Records have variable length but cannot cross block boundaries.
Cost: If a record does not fit in the block remainder, that space is wasted and the record starts in the next block.