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block suballocation
Block suballocation is a feature of some computer file systems which allows large blocks or allocation units to be used while making efficient use of "slack" space at the end of large files, space which would otherwise be lost for other use to internal fragmentation.〔 (Fundamental patent.)〕
In file systems that don't support fragments, this feature is also called tail merging or tail packing because it is commonly done by packing the "tail", or last partial block, of multiple files into a single block.
== 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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