Ubuntu Manpage: fallocate - manipulate file space

NAME

       fallocate - manipulate file space

SYNOPSIS

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <fcntl.h>

       int fallocate(int fd, int mode, off_t offset, off_t len);

DESCRIPTION

       This  is  a  nonportable, Linux-specific system call.  For the portable, POSIX.1-specified
       method of ensuring that space is allocated for a file, see posix_fallocate(3).

       fallocate() allows the caller to directly manipulate the allocated disk space for the file
       referred to by fd for the byte range starting at offset and continuing for len bytes.

       The mode argument determines the operation to be performed on the given range.  Details of
       the supported operations are given in the subsections below.

   Allocating disk space
       The default operation (i.e., mode is zero) of fallocate() allocates the disk space  within
       the  range  specified  by  offset and len.  The file size (as reported by stat(2)) will be
       changed if offset+len is greater than the file  size.   Any  subregion  within  the  range
       specified  by offset and len that did not contain data before the call will be initialized
       to zero.  This default behavior closely resembles the behavior of  the  posix_fallocate(3)
       library function, and is intended as a method of optimally implementing that function.

       After  a successful call, subsequent writes into the range specified by offset and len are
       guaranteed not to fail because of lack of disk space.

       If the FALLOC_FL_KEEP_SIZE flag is specified in mode, the behavior of the call is similar,
       but  the  file  size will not be changed even if offset+len is greater than the file size.
       Preallocating zeroed blocks beyond the end of the  file  in  this  manner  is  useful  for
       optimizing append workloads.

       Because  allocation  is done in block size chunks, fallocate() may allocate a larger range
       of disk space than was specified.

   Deallocating file space
       Specifying  the  FALLOC_FL_PUNCH_HOLE  flag  (available  since  Linux  2.6.38)   in   mode
       deallocates  space  (i.e.,  creates  a  hole)  in  the  byte  range starting at offset and
       continuing for len 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.

       The FALLOC_FL_PUNCH_HOLE flag must be ORed with  FALLOC_FL_KEEP_SIZE  in  mode;  in  other
       words,  even when punching off the end of the file, the file size (as reported by stat(2))
       does not change.

       Not all filesystems support FALLOC_FL_PUNCH_HOLE; if  a  filesystem  doesn't  support  the
       operation,  an  error  is  returned.  The operation is supported on at least the following
       filesystems:

       *  XFS (since Linux 2.6.38)

       *  ext4 (since Linux 3.0)

       *  Btrfs (since Linux 3.7)

       *  tmpfs (since Linux 3.5)

   Collapsing file space
       Specifying the FALLOC_FL_COLLAPSE_RANGE flag (available since Linux 3.15) in mode  removes
       a  byte  range from a file, without leaving a hole.  The byte range to be collapsed starts
       at offset and continues for len bytes.  At the completion of the operation,  the  contents
       of  the  file starting at the location offset+len will be appended at the location offset,
       and the file will be len bytes smaller.

       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, fallocate() will fail with the error EINVAL if this
       requirement is violated.

       If the region specified by offset plus len reaches or passes the end of file, an error  is
       returned; instead, use ftruncate(2) to truncate a file.

       No other flags may be specified in mode in conjunction with FALLOC_FL_COLLAPSE_RANGE.

       As  at  Linux  3.15,  FALLOC_FL_COLLAPSE_RANGE is supported by ext4 (only for extent-based
       files) and XFS.

   Zeroing file space
       Specifying the FALLOC_FL_ZERO_RANGE flag (available since Linux 3.15) in mode zeroes space
       in  the  byte range starting at offset and continuing for len 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.

       If the FALLOC_FL_KEEP_SIZE flag is additionally specified in mode,  the  behavior  of  the
       call  is similar, but the file size will not be changed even if offset+len is greater than
       the  file  size.   This  behavior  is  the  same  as   when   preallocating   space   with
       FALLOC_FL_KEEP_SIZE specified.

       Not  all  filesystems  support  FALLOC_FL_ZERO_RANGE;  if a filesystem doesn't support the
       operation, an error is returned.  The operation is supported on  at  least  the  following
       filesystems:

       *  XFS (since Linux 3.15)

       *  ext4, for extent-based files (since Linux 3.15)

       *  SMB3 (since Linux 3.17)

   Increasing file space
       Specifying  the  FALLOC_FL_INSERT_RANGE flag (available since Linux 4.1) in mode increases
       the file space by inserting a hole within the file size without overwriting  any  existing
       data.   The hole will start at offset and continue for len bytes.  When inserting the hole
       inside file, the contents of the file starting at offset will be shifted upward (i.e.,  to
       a  higher  file  offset)  by len bytes.  Inserting a hole inside a file increases the file
       size by len bytes.

       This mode has the same limitations as FALLOC_FL_COLLAPSE_RANGE regarding  the  granularity
       of the operation.  If the granularity requirements are not met, fallocate() will fail with
       the error EINVAL.  If the offset is equal to or greater than the end of file, an error  is
       returned.   For  such operations (i.e., inserting a hole at the end of file), ftruncate(2)
       should be used.

       No other flags may be specified in mode in conjunction with FALLOC_FL_INSERT_RANGE.

       FALLOC_FL_INSERT_RANGE  requires  filesystem  support.   Filesystems  that  support   this
       operation include XFS (since Linux 4.1) and ext4 (since Linux 4.2).

RETURN VALUE

       On  success,  fallocate()  returns  zero.   On  error,  -1 is returned and errno is set to
       indicate the error.

ERRORS

EBADF fd is not a valid file descriptor, or is not opened for writing.

EFBIG offset+len exceeds the maximum file size.

EFBIG mode is FALLOC_FL_INSERT_RANGE, and the current file size+len exceeds the maximum
file size.

EINTR A signal was caught during execution.

EINVAL offset was less than 0, or len was less than or equal to 0.

EINVAL mode is FALLOC_FL_COLLAPSE_RANGE and the range specified by offset plus len reaches
or passes the end of the file.

EINVAL mode is FALLOC_FL_INSERT_RANGE and the range specified by offset reaches or passes
the end of the file.

EINVAL mode is FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE, but either offset or
len is not a multiple of the filesystem block size.

EINVAL mode contains one of FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE and also
other flags; no other flags are permitted with FALLOC_FL_COLLAPSE_RANGE or
FALLOC_FL_INSERT_RANGE.

EINVAL mode is FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_ZERO_RANGE or FALLOC_FL_INSERT_RANGE,
but the file referred to by fd is not a regular file.

EIO An I/O error occurred while reading from or writing to a filesystem.

ENODEV fd does not refer to a regular file or a directory. (If fd is a pipe or FIFO, a
different error results.)

ENOSPC There is not enough space left on the device containing the file referred to by fd.

ENOSYS This kernel does not implement fallocate().

EOPNOTSUPP
The filesystem containing the file referred to by fd does not support this
operation; or the mode is not supported by the filesystem containing the file
referred to by fd.

EPERM The file referred to by fd is marked immutable (see chattr(1)).

EPERM mode specifies FALLOC_FL_PUNCH_HOLE or FALLOC_FL_COLLAPSE_RANGE or
FALLOC_FL_INSERT_RANGE and the file referred to by fd is marked append-only (see
chattr(1)).

EPERM The operation was prevented by a file seal; see fcntl(2).

ESPIPE fd refers to a pipe or FIFO.

ETXTBSY
mode specifies FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE, but the file
referred to by fd is currently being executed.

VERSIONS

       fallocate() is available on Linux since kernel 2.6.23.  Support is provided by glibc since
       version 2.10.  The FALLOC_FL_* flags are defined in glibc headers only since version 2.18.

CONFORMING TO

       fallocate() is Linux-specific.

COLOPHON

       This page is part of release 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.