Block Suballocation - Rationale

Rationale

File systems have traditionally divided the disk into equally sized blocks to simplify their design and limit the worst-case fragmentation. Block sizes are typically multiples of 512 due to the size of hard disk sectors. When files are allocated by some traditional file systems, only whole blocks can be allocated to individual files. But as file sizes are often not multiples of the file system block size, this design inherently results in the last blocks of files (called tails) occupying only a part of the block, resulting in what is called internal fragmentation (not to be confused with external fragmentation). This waste of space can be significant if the file system stores many small files and can become critical when attempting to use higher block sizes to improve performance. FFS and other derived UNIX file systems support fragments which greatly mitigate this effect.