Contiguous Allocation

Each file occupies set of contiguous blocks Best performance in most cases Simple – only starting location (block #) and length (number of blocks) are required Problems include finding space for file, knowing file size, external fragmentation, need for compaction off-line (downtime) or on-line

Linked allocation

Each file a linked list of blocks File ends at nil pointer No external fragmentation Each block contains pointer to next block No compaction, external fragmentation Free space management system called when new block needed Improve efficiency by clustering blocks into groups but increases internal fragmentation Reliability can be a problem Locating a block can take many I/Os and disk seeks

Indexed allocation

Each file has its own index block(s) of pointers to its data blocks

Need index table Random access Dynamic access without external fragmentation, but have overhead of index block Mapping from logical to physical in a file of maximum size of 256K bytes and block size of 512 bytes. We need only 1 block for index table