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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 spec‐
ified 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 opti‐
mizing 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 deallo‐
cates 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 (oth‐
erwise) 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 FAL‐
LOC_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 indi‐
cate 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 opera‐
tion; 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 cur‐
rently 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 3.74 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 http://www.kernel.org/doc/man-pages/.
Linux 2014-07-08 FALLOCATE(2)
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