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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 partially empty. This is achieved by writing brief information (metadata) representing the empty blocks to the data storage media instead of the actual "empty" space which makes up the block, thus consuming less storage space. The full block is written to the media as the actual size only when the block contains "real" (non-empty) data.
Most commonly, sparse files are created when blocks of the file are never written to. This is typical for random-access files like databases. Some operating systems or utilities go further by "sparsifying" files when writing or copying them: if a block contains only null bytes, it is not written to storage but rather marked as empty.
When reading sparse files, the file system transparently converts metadata representing empty blocks into "real" blocks filled with null bytes at runtime. The application is unaware of this conversion.
Most modern file systems support sparse files, including most Unix variants and NTFS. Apple's HFS+ does not provide support for sparse files, but in OS X, the virtual file system layer supports storing them in any supported file system, including HFS+. Apple File System (APFS) also supports them. Sparse files are commonly used for disk images, database snapshots, log files and in scientific applications.
For example, a virtual machine image with max size of 100 GB that has 2 GB of files actually written would require the full 100 GB when backed by pre-allocated storage, yet only 2 GB on a sparse file. If the file system supports hole punching and the guest operating system issues TRIM commands, deleting files on the guest will accordingly reduce the space needed.
Similarly the truncate command may be used, if available:
On Linux, an existing file can be converted to sparse by:
There is no portable system call to punch holes; Linux provides fallocate(FALLOC_FL_PUNCH_HOLE), and Solaris provides fcntl(F_FREESP).
Alternatively, the du command prints the occupied space, while ls prints the apparent size.
In some non-standard versions of du, the option prints the occupied space in bytes instead of blocks, so that it can be compared to the --block-size=1 output:
Note the above du usage has the abbreviated option syntax format "du -B 1 sf", itself equivalent to the shortest version "du -b sf" as stated in the du manual: is equivalent to .
Also, the tool ls from filefrag package can be used to show block allocation details of the file.
cp sparse-file new-file
creates new-file, which will be sparse. GNU also has a cp option,
which is especially useful if a file containing long zero blocks is saved in a non-sparse way (i.e. the zero blocks have been written to the storage media in full). Storage space can be conserved by doing:
cp --sparse=always file1 file1_sparsed
Some cp implementations, like FreeBSD's cp, do not support the --sparse option and will always expand sparse files. A partially viable alternative on those systems is to use rsync with its own --sparse option
instead of cp. Unfortunately, older versions of rsync do not support --sparse combined with --sparse.
Via standard input, sparse file copying is achieved as follows:
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