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FALLOCATE

Section: Linux Programmer's Manual (2)
Updated: 2015-01-22
Index Return to Main Contents
 

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)).
EPERM
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)).
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, 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 3.81 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/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
Allocating disk space
Deallocating file space
Collapsing file space
Zeroing file space
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
SEE ALSO
COLOPHON


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