File system vs virtual memory; contiguous allocation

File system management versus virtual memory management

They are similar in the following ways:

• Both give a logical view (file/virtual address) independent of physical location, with a mapping (directory/i-node or page table) to physical blocks/frames.
• Both use fixed-size units (disk blocks / pages and frames) and have allocation methods and free-space lists (bitmap, free list).
• Both use indexed/multi-level structures (i-node indirect blocks and multi-level page tables).
• Both use caching (buffer cache / TLB and page cache) and suffer fragmentation (internal in the last block/page).
• Both support sharing and protection and use secondary storage (disk) with a replacement policy for the cache.

Contiguous allocation scheme

Each file occupies consecutive disk blocks.

• Advantages: simple; the directory stores only the start block and length; excellent read performance (one seek for the whole file) for both sequential and direct access.
• Disadvantages: external fragmentation (requires compaction); the file size must be known when created; hard to grow; wasted space if reserved size is large.

File organization best for tape

Sequential (contiguous) organization is most suitable for tape. A tape is a sequential-access device: it can only be read from the start in order, and moving to a given block requires winding the tape. Contiguous storage lets records be read and written in order at full tape speed with no seeking or pointer chasing; linked and indexed methods rely on random access and would be extremely slow.