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sparse file
In computer science, a sparse file is a type of computer file that attempts to use file system space more efficiently when the file itself is mostly empty. This is achieved by writing brief information (metadata) ''representing'' the empty blocks to disk instead of the actual "empty" space which makes up the block, using less disk space. The full block size is written to disk as the actual size only when the block contains "real" (non-empty) data.
When reading sparse files, the file system transparently converts metadata representing empty blocks into "real" blocks filled with zero bytes at runtime. The application is unaware of this conversion.
Most modern file systems support sparse files, including most Unix variants and NTFS.〔Giampaolo, Dominic (1999). ''(Practical File System Design with the Be File System )''. Morgan Kaufmann. ISBN 1-55860-497-9〕 Apple's HFS+ does not provide for sparse files, but in OS X, the virtual file system layer supports storing them in any supported file system, including HFS+. Sparse files are commonly used for disk images, database snapshots, log files and in scientific applications.
==Advantages==
The advantage of sparse files is that storage is only allocated when actually needed: disk space is saved, and large files can be created even if there is insufficient free space on the file system. This also reduces the time of the first write as the system doesn't have to allocate blocks for the "skipped" space. Since initial allocation usually requires the system to write all zeros to the space, it also keeps the system from having to write over the "skipped" space twice.
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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