Manpage of FALLOCATE

FALLOCATE

Section: Linux Programmer's Manual (2)
Updated: 2016-12-12
Index
 

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 fdfor the byte range starting at offsetand continuing for lenbytes.

The modeargument 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., modeis zero) of fallocate() allocates the disk space within the range specified by offsetand len. The file size (as reported by stat(2)) will be changed if offset+lenis greater than the file size. Any subregion within the range specified by offsetand lenthat 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 offsetand lenare guaranteed not to fail because of lack of disk space.

If the FALLOC_FL_KEEP_SIZEflag is specified in mode, the behavior of the call is similar, but the file size will not be changed even if offset+lenis 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_UNSHAREflag 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_HOLEflag (available since Linux 2.6.38) in modedeallocates space (i.e., creates a hole) in the byte range starting at offsetand continuing for lenbytes. 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_HOLEflag must be ORed with FALLOC_FL_KEEP_SIZEin 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)
 

Collapsing file space

Specifying the FALLOC_FL_COLLAPSE_RANGEflag (available since Linux 3.15) in moderemoves a byte range from a file, without leaving a hole. The byte range to be collapsed starts at offsetand continues for lenbytes. At the completion of the operation, the contents of the file starting at the location offset+lenwill be appended at the location offset, and the file will be lenbytes smaller.

A filesystem may place limitations on the granularity of the operation, in order to ensure efficient implementation. Typically, offsetand lenmust 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 EINVALif this requirement is violated.

If the region specified by offsetplus lenreaches 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 modein conjunction with FALLOC_FL_COLLAPSE_RANGE.

As at Linux 3.15, FALLOC_FL_COLLAPSE_RANGEis supported by ext4 (only for extent-based files) and XFS.  

Zeroing file space

Specifying the FALLOC_FL_ZERO_RANGEflag (available since Linux 3.15) in modezeroes space in the byte range starting at offsetand continuing for lenbytes. 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_SIZEflag is additionally specified in mode, the behavior of the call is similar, but the file size will not be changed even if offset+lenis greater than the file size. This behavior is the same as when preallocating space with FALLOC_FL_KEEP_SIZEspecified.

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)
 

Increasing file space

Specifying the FALLOC_FL_INSERT_RANGEflag (available since Linux 4.1) in modeincreases the file space by inserting a hole within the file size without overwriting any existing data. The hole will start at offsetand continue for lenbytes. When inserting the hole inside file, the contents of the file starting at offsetwill be shifted upward (i.e., to a higher file offset) by lenbytes. Inserting a hole inside a file increases the file size by lenbytes.

This mode has the same limitations as FALLOC_FL_COLLAPSE_RANGEregarding the granularity of the operation. If the granularity requirements are not met, fallocate() will fail with the error EINVAL.If the offsetis 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 modein conjunction with FALLOC_FL_INSERT_RANGE.

FALLOC_FL_INSERT_RANGErequires 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 errnois set to indicate the error.  

ERRORS

EBADF
fdis not a valid file descriptor, or is not opened for writing.
EFBIG
offset+lenexceeds the maximum file size.
EFBIG
modeis 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
offsetwas less than 0, or lenwas less than or equal to 0.
EINVAL
modeis FALLOC_FL_COLLAPSE_RANGEand the range specified by offsetplus lenreaches or passes the end of the file.
EINVAL
modeis FALLOC_FL_INSERT_RANGEand the range specified by offsetreaches or passes the end of the file.
EINVAL
modeis FALLOC_FL_COLLAPSE_RANGEor FALLOC_FL_INSERT_RANGE, but either offsetor lenis not a multiple of the filesystem block size.
EINVAL
modecontains one of FALLOC_FL_COLLAPSE_RANGEor FALLOC_FL_INSERT_RANGEand also other flags; no other flags are permitted with FALLOC_FL_COLLAPSE_RANGEor FALLOC_FL_INSERT_RANGE.
EINVAL
modeis FALLOC_FL_COLLAPSE_RANGEor FALLOC_FL_ZERO_RANGEor FALLOC_FL_INSERT_RANGE, but the file referred to by fdis not a regular file.
EIO
An I/O error occurred while reading from or writing to a filesystem.
ENODEV
fddoes not refer to a regular file or a directory. (If fdis 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 fddoes not support this operation; or the modeis not supported by the filesystem containing the file referred to by fd.
EPERM
The file referred to by fdis marked immutable (see chattr(1)).
EPERM
modespecifies FALLOC_FL_PUNCH_HOLEor FALLOC_FL_COLLAPSE_RANGEor FALLOC_FL_INSERT_RANGEand the file referred to by fdis marked append-only (see chattr(1)).
EPERM
The operation was prevented by a file seal; see fcntl(2).
ESPIPE
fdrefers to a pipe or FIFO.
ETXTBSY
modespecifies FALLOC_FL_COLLAPSE_RANGEor FALLOC_FL_INSERT_RANGE, but the file referred to by fdis 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
Increasing file space
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
SEE ALSO

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