FALLOCATE
Section: Linux Programmer's Manual (2)
Updated: 2019-11-19
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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.
SEE ALSO
fallocate(1),
ftruncate(2),
posix_fadvise(3),
posix_fallocate(3)
COLOPHON
This page is part of release 5.11 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/.
Index
- 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
-
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Time: 06:22:44 GMT, May 09, 2021