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
       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.

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

COLOPHON
       This page is part of release 5.04 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
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-11-19                      FALLOCATE(2)