Linux man pages

Section (1) fallocate

Linux manual pages	Section 1

Name

fallocate — preallocate or deallocate space to a file

Synopsis

fallocate [ −c | −p | −z ] [ −o offset ] −l length [−n] filename

fallocate -d [ −o offset ] [ −l length ] filename

fallocate -x [ −o offset ] −l length filename

DESCRIPTION

fallocate is used to manipulate the allocated disk space for a file, either to deallocate or preallocate it. For filesystems which support the fallocate system call, preallocation is done quickly by allocating blocks and marking them as uninitialized, requiring no IO to the data blocks. This is much faster than creating a file by filling it with zeroes.

The exit code returned by fallocate is 0 on success and 1 on failure.

OPTIONS

The length and offset arguments may be followed by the multiplicative suffixes KiB (=1024), MiB (=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB, and YiB (the iB is optional, e.g., K has the same meaning as KiB) or the suffixes KB (=1000), MB (=1000*1000), and so on for GB, TB, PB, EB, ZB, and YB.

The options −−collapse−range, −−dig−holes, −−punch−hole, and −−zero−range are mutually exclusive.

−c, −−collapse−range

Removes a byte range from a file, without leaving a hole. The byte range to be collapsed starts at offset and continues for length bytes. At the completion of the operation, the contents of the file starting at the location offset+length will be appended at the location offset, and the file will be length bytes smaller. The option −−keep−size may not be specified for the collapse-range operation.

Available since Linux 3.15 for ext4 (only for extent-based files) and XFS.

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, the operation will fail with the error EINVAL if this requirement is violated.

−d, −−dig−holes

Detect and dig holes. This makes the file sparse in-place, without using extra disk space. The minimum size of the hole depends on filesystem I/O block size (usually 4096 bytes). Also, when using this option, −−keep−size is implied. If no range is specified by −−offset and −−length, then the entire file is analyzed for holes.

You can think of this option as doing a cp −−sparse and then renaming the destination file to the original, without the need for extra disk space.

See −−punch−hole for a list of supported filesystems.

−i, −−insert−range

Insert a hole of length bytes from offset, shifting existing data.

−l, −−length length

Specifies the length of the range, in bytes.

−n, −−keep−size

Do not modify the apparent length of the file. This may effectively allocate blocks past EOF, which can be removed with a truncate.

−o, −−offset offset

Specifies the beginning offset of the range, in bytes.

−p, −−punch−hole

Deallocates space (i.e., creates a hole) in the byte range starting at offset and continuing for length 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 zeroes. This option may not be specified at the same time as the −−zero−range option. Also, when using this option, −−keep−size is implied.

Supported for XFS (since Linux 2.6.38), ext4 (since Linux 3.0), Btrfs (since Linux 3.7) and tmpfs (since Linux 3.5).

−v, −−verbose

Enable verbose mode.

−x, −−posix

Enable POSIX operation mode. In that mode allocation operation always completes, but it may take longer time when fast allocation is not supported by the underlying filesystem.

−z, −−zero−range

Zeroes space in the byte range starting at offset and continuing for length 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 zeroes.

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.

Option −−keep−size can be specified to prevent file length modification.

Available since Linux 3.14 for ext4 (only for extent-based files) and XFS.

−V, −−version

Display version information and exit.

−h, −−help

Display help text and exit.

AUTHORS

Eric Sandeen

Karel Zak

AVAILABILITY

The fallocate command is part of the util-linux package and is available from Linux Kernel Archive

Section (2) fallocate

Linux manual pages	Section 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_zsingle_quotesz_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.

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_zsingle_quotesz_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

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/.

Copyright (c) 2007 Silicon Graphics, Inc. All Rights Reserved
Written by Dave Chinner <dgcsgi.com>

%%%LICENSE_START(GPLv2_ONELINE)
May be distributed as per GNU General Public License version 2.
%%%LICENSE_END

2011-09-19: Added FALLOC_FL_PUNCH_HOLE
2011-09-19: Substantial restructuring of the page

Linux man pages

Section (1) fallocate

Name

Synopsis

DESCRIPTION

OPTIONS

AUTHORS

SEE ALSO

AVAILABILITY

Section (2) fallocate

Name

Synopsis

DESCRIPTION

Allocating disk space

Deallocating file space

Collapsing file space

Zeroing file space

Increasing file space

RETURN VALUE

ERRORS

VERSIONS

CONFORMING TO

SEE ALSO

COLOPHON