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man:fallocate

FALLOCATE(2) Linux Programmer's Manual FALLOCATE(2)

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  subsec-
     tions 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 off-
     set+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 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  allo-
     cate 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)
 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  FAL-
     LOC_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  par-
     tial  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 regard-
     ing 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), ftrun-
     cate(2) should be used.
     No other flags may be  specified  in  mode  in  conjunction  with  FAL-
     LOC_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 off-
            set 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   FAL-
            LOC_FL_INSERT_RANGE  and  also  other  flags; no other flags are
            permitted     with     FALLOC_FL_COLLAPSE_RANGE     or      FAL-
            LOC_FL_INSERT_RANGE.
     EINVAL mode is FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_ZERO_RANGE or FAL-
            LOC_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     FAL-
            LOC_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 pro-
     vided 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.

SEE ALSO

     fallocate(1), ftruncate(2), posix_fadvise(3), posix_fallocate(3)

COLOPHON

     This  page  is  part of release 4.16 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
     https://www.kernel.org/doc/man-pages/.

Linux 2018-04-30 FALLOCATE(2)

/data/webs/external/dokuwiki/data/pages/man/fallocate.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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