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FALLOCATE(1) User Commands FALLOCATE(1)
NAME
fallocate - preallocate or deallocate space to a file
SYNOPSIS
fallocate [-c|-p|-z] [-o offset] -l length [-n] filename
fallocate -d [-o offset] [-l length] filename
fallocate -x [-o offset] -l length filename
DESCRIPTION
fallocate is used to manipulate the allocated disk space for a file, either to deallocate
or preallocate it. For filesystems which support the fallocate 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.
OPTIONS
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, and --zero-range 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 len 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.
-V, --version
Display version information and exit.
-h, --help
Display help text and exit.
AUTHORS
Eric Sandeen <sandeen AT redhat.com>, Karel Zak <kzak AT redhat.com>
SEE ALSO
truncate(1), fallocate(2), posix_fallocate(3)
REPORTING BUGS
For bug reports, use the issue tracker at https://github.com/karelzak/util-linux/issues.
AVAILABILITY
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/>.
util-linux 2.37.2 2021-06-02 FALLOCATE(1)
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