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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | AUTHORS | SEE ALSO | REPORTING BUGS | AVAILABILITY |
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FALLOCATE(1) User Commands FALLOCATE(1)
fallocate - preallocate or deallocate space to a file
fallocate [-c|-p|-z] [-o offset] -l length [-n] filename
fallocate -d [-o offset] [-l length] filename
fallocate -x [-o offset] -l length filename
fallocate is used to manipulate the allocated disk space for a
file, either to deallocate or preallocate it. For filesystems
which support the fallocate(2) system call, preallocation is done
quickly by allocating blocks and marking them as uninitialized,
requiring no IO to the data blocks. This is much faster than
creating a file by filling it with zeroes.
The exit status returned by fallocate is 0 on success and 1 on
failure.
The length and offset arguments may be followed by the
multiplicative suffixes KiB (=1024), MiB (=1024*1024), and so on
for GiB, TiB, PiB, EiB, ZiB, and YiB (the "iB" is optional, e.g.,
"K" has the same meaning as "KiB") or the suffixes KB (=1000), MB
(=1000*1000), and so on for GB, TB, PB, EB, ZB, and YB.
The options --collapse-range, --dig-holes, --punch-hole,
--zero-range and --posix are mutually exclusive.
-c, --collapse-range
Removes a byte range from a file, without leaving a hole. The
byte range to be collapsed starts at offset and continues for
length bytes. At the completion of the operation, the contents
of the file starting at the location offset+length will be
appended at the location offset, and the file will be length
bytes smaller. The option --keep-size may not be specified for
the collapse-range operation.
Available since Linux 3.15 for ext4 (only for extent-based
files) and XFS.
A filesystem may place limitations on the granularity of the
operation, in order to ensure efficient implementation.
Typically, offset and length must be a multiple of the
filesystem logical block size, which varies according to the
filesystem type and configuration. If a filesystem has such a
requirement, the operation will fail with the error EINVAL if
this requirement is violated.
-d, --dig-holes
Detect and dig holes. This makes the file sparse in-place,
without using extra disk space. The minimum size of the hole
depends on filesystem I/O block size (usually 4096 bytes).
Also, when using this option, --keep-size is implied. If no
range is specified by --offset and --length, then the entire
file is analyzed for holes.
You can think of this option as doing a "cp --sparse" and then
renaming the destination file to the original, without the
need for extra disk space.
See --punch-hole for a list of supported filesystems.
-i, --insert-range
Insert a hole of length bytes from offset, shifting existing
data.
-l, --length length
Specifies the length of the range, in bytes.
-n, --keep-size
Do not modify the apparent length of the file. This may
effectively allocate blocks past EOF, which can be removed
with a truncate.
-o, --offset offset
Specifies the beginning offset of the range, in bytes.
-p, --punch-hole
Deallocates space (i.e., creates a hole) in the byte range
starting at offset and continuing for length bytes. Within the
specified range, partial filesystem blocks are zeroed, and
whole filesystem blocks are removed from the file. After a
successful call, subsequent reads from this range will return
zeroes. This option may not be specified at the same time as
the --zero-range option. Also, when using this option,
--keep-size is implied.
Supported for XFS (since Linux 2.6.38), ext4 (since Linux
3.0), Btrfs (since Linux 3.7), tmpfs (since Linux 3.5) and
gfs2 (since Linux 4.16).
-v, --verbose
Enable verbose mode.
-x, --posix
Enable POSIX operation mode. In that mode allocation operation
always completes, but it may take longer time when fast
allocation is not supported by the underlying filesystem.
-z, --zero-range
Zeroes space in the byte range starting at offset and
continuing for length bytes. Within the specified range,
blocks are preallocated for the regions that span the holes in
the file. After a successful call, subsequent reads from this
range will return zeroes.
Zeroing is done within the filesystem preferably by converting
the range into unwritten extents. This approach means that the
specified range will not be physically zeroed out on the
device (except for partial blocks at the either end of the
range), and I/O is (otherwise) required only to update
metadata.
Option --keep-size can be specified to prevent file length
modification.
Available since Linux 3.14 for ext4 (only for extent-based
files) and XFS.
-h, --help
Display help text and exit.
-V, --version
Display version and exit.
Eric Sandeen <[email protected]>, Karel Zak <[email protected]>
truncate(1), fallocate(2), posix_fallocate(3)
For bug reports, use the issue tracker
<https://github.com/util-linux/util-linux/issues>.
The fallocate command is part of the util-linux package which can
be downloaded from Linux Kernel Archive
<https://www.kernel.org/pub/linux/utils/util-linux/>. This page is
part of the util-linux (a random collection of Linux utilities)
project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
bug report for this manual page, send it to
[email protected]. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2025-08-11. (At that time, the date of the most recent commit that
was found in the repository was 2025-08-05.) If you discover any
rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to [email protected]
util-linux 2.42-start-521-ec46 2025-02-02 FALLOCATE(1)
Pages that refer to this page: fallocate(2), posix_fallocate(3), swapon(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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