fallocate

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@redhat.com>, Karel Zak <kzak@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)