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.

STANDARDS

       Linux.

HISTORY

       fallocate()
              Linux 2.6.23, glibc 2.10.

       FALLOC_FL_*
              glibc 2.18.