fallocate(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | SEE ALSO | COLOPHON

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

NAME         top

       fallocate - manipulate file space

SYNOPSIS         top

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <fcntl.h>

       int fallocate(int fd, int mode, off_t offset, off_t len);

DESCRIPTION         top

       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         top

       On success, fallocate() returns zero.  On error, -1 is returned
       and errno is set to indicate the error.

ERRORS         top

       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         top

       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         top

       fallocate() is Linux-specific.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 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)

Pages that refer to this page: fallocate(1)rsync(1)fcntl(2)ioctl_ficlonerange(2)ioctl_fideduperange(2)lseek(2)madvise(2)syscalls(2)posix_fallocate(3)fanotify(7)inotify(7)xfs_io(8)