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
Advantages
The advantage of sparse files is that storage space is only allocated when actually needed: Storage capacity is conserved, and large files can occasionally be created even if insufficient free space for the original file is available on the storage media. This also reduces the time of the first write as the system does not have to allocate blocks for the "skipped" space. If the initial allocation requires writing all zeros to the space, it also keeps the system from having to write over the "skipped" space twice.
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.
Disadvantages
Disadvantages are that sparse files may become
Sparse files in Unix
Sparse files are typically handled transparently to the user. But the differences between a normal file and sparse file become apparent in some situations.
Creation
The Unix command
dd of=sparse-file bs=5M seek=1 count=0
will create a file of five
has this behavior because it callsftruncate
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 <filename>
On Linux, an existing file can be converted to sparse by:
fallocate -d <filename>
There is no portable system call to punch holes; Linux provides fallocate(FALLOC_FL_PUNCH_HOLE)
, and
fcntl(F_FREESP)
.
Detection
The -s
option of the ls
command shows the occupied space in blocks.
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 {{{1}}} 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:[3] -b, --bytes
is equivalent to --apparent-size --block-size=1
.
Also, the tool filefrag
from e2fsprogs
package can be used to show block allocation details of the file.
filefrag -v sparse-file
Copying
Normally the GNU version of cp
is good at detecting whether a file is sparse, so
cp sparse-file new-file
creates new-file, which will be sparse. However, GNU cp does have a --sparse
option.[4] This 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[5] instead of cp. Unfortunately --sparse
cannot be combined with --inplace
.[6][7] Newer Versions of rsync do support --sparse
combined with --inplace
.[8]
Via
cp --sparse=always /dev/fd/0 new-sparse-file < somefile
See also
References
- ^
ISBN 9781558604971.
- ^ "Apple File System Guide". Apple's Developer Site. Apple Inc. Retrieved 27 April 2017.
- ^ "Du(1) – Linux manual page".
- ^ Meyering, Jim (1995-12-21). "GNU coreutils/cp: Accept new option, --sparse={never,auto,always}, to control creation of sparse files". Retrieved 2016-06-17.
- ^ Tridgell, Andrew (1996-06-29). "rsync: hard links, better sparse handling, FERROR and FINFO". Retrieved 2016-06-17.
- ^ Tridgell, Andrew (2016-06-30). "rsync manpage". Retrieved 2017-01-19.
- ^ Davison, Wayne (2005-08-30). "rsync: Reject attempts to combine --sparse with --inplace". Retrieved 2017-01-19.
- ^ Davison, Wayne. "Support --sparse combined with --preallocate or --inplace".
External links
- NTFS Sparse Files For Programmers
- 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