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 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  ap-
              pended 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 op-
              eration,  in  order  to  ensure efficient implementation.  Typi-
              cally, offset and len must be a multiple of the filesystem logi-
              cal  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  effec-
              tively  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) 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  alloca-
              tion 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 preal-
              located  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 modi-
              fication.

              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)

AVAILABILITY
       The  fallocate  command is part of the util-linux package and is avail-
       able from Linux Kernel Archive  <https://www.kernel.org/pub/linux/utils
       /util-linux/>.

util-linux                        April 2014                      FALLOCATE(1)