FALLOCATE
Section: Linux Programmer's Manual (2)
Updated: 2014-07-08
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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.
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 zeroes.
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 (since Linux 3.5)
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()
will fail 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 3.14)
in
mode
zeroes 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 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.
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 3.14)
- *
-
ext4, for extent-based files (since Linux 3.14)
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.
- EINTR
-
A signal was caught during execution.
- 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_COLLAPSE_RANGE,
but either
offset
or
len
is not a multiple of the filesystem block size.
- EINVAL
-
mode
contains both
FALLOC_FL_COLLAPSE_RANGE
and other flags;
no other flags are permitted with
FALLOC_FL_COLLAPSE_RANGE.
- EINVAL
-
mode
is
FALLOC_FL_COLLAPSE_RANGE
or
FALLOC_FL_ZERO_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)).
Or:
mode
specifies
FALLOC_FL_PUNCH_HOLE
or
FALLOC_FL_COLLAPSE_RANGE
and
the file referred to by
fd
is marked append-only
(see
chattr(1)).
- ESPIPE
-
fd
refers to a pipe or FIFO.
- ETXTBSY
-
mode
specifies
FALLOC_FL_COLLAPSE_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)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Allocating disk space
-
- Deallocating file space
-
- Collapsing file space
-
- Zeroing file space
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- SEE ALSO
-
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Time: 02:54:50 GMT, September 18, 2014