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 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.…

.]] 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. dd of=sparse-file bs=5M seek=1 count=0 will create a file of five mebibytes in size, but with no data stored on the media (only metadata). (GNU dd has this behavior because it calls ftruncate to set the file size; other implementations may merely create an empty file.) Similarly the truncate command may be used, if available: truncate -s 5M On Linux, an existing file can be converted to sparse by: fallocate -d There is no portable system call to punch holes; Linux provides fallocate(FALLOC_FL_PUNCH_HOLE) , and Solaris provides fcntl(F_FREESP) . ls -ls sparse-file 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 ls output: du --block-size=1 sparse-file ls -l sparse-file 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 filefrag from e2fsprogs package can be used to show block allocation details of the file. filefrag -v sparse-file 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 --inplace . Via standard input, sparse file copying is achieved as follows: cp --sparse=always /dev/fd/0 new-sparse-file Creating sparse files in Windows Server using fsutil Creating sparse files in Solaris using mkfile(1M) View the Size of the Sparse File of a Database Snapshot SEEK_HOLE or FIEMAP: Detecting holes in sparse files virtsync is a commercial solution to rsync's --sparse and --inplace issue. SparseChecker – Utility that allows to manage the sparse files on NTFS file system Phantom – a program to convert files to sparse files to reduce storage consumption ArchLinux Wiki: Sparse file

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