Ubuntu Manpage: fallocate - manipulate file space

NAME

       fallocate - manipulate file space

LIBRARY

       Standard C library (libc, -lc)

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.

       If the FALLOC_FL_UNSHARE_RANGE flag is specified in mode, shared file data extents will be
       made private to the file to guarantee that a subsequent write will not fail due to lack of
       space.  Typically, this will be done by performing a copy-on-write operation on all shared
       data in the file.  This flag may not be supported by all filesystems.

       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 zeros.

       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(5) (since Linux 3.5)

       •  gfs2(5) (since Linux 4.16)

   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()  fails  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 zeros 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 zeros.

       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)

       •  Btrfs (since Linux 4.16)

   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() fails 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; see signal(7).

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, 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, 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 since Linux 2.6.23.  Support is provided since glibc  2.10.   The
       FALLOC_FL_* flags are defined in glibc headers only since glibc 2.18.

STANDARDS

       fallocate() is Linux-specific.