Ubuntu Manpage: fallocate - manipulate file space

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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.

       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 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 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

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
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Linux                                              2019-11-19                                       FALLOCATE(2)