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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 code 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.
-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 desti-
nation 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 suc-
cessful 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) and tmpfs (since Linux 3.5).
-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 physi-
cally 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)
AVAILABILITY
The fallocate command is part of the util-linux package and is available from Linux Kernel
Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.
util-linux April 2014 FALLOCATE(1)
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