Section (1) fallocate
Name
fallocate — preallocate or deallocate space to a file
Synopsis
fallocate
[ −c
| −p
| −z
] [ −o
offset ] −l
length [−n
] filename
fallocate -d
[ −o
offset ] [ −l
length ] filename
fallocate -x
[ −o
offset ] −l
length filename
DESCRIPTION
fallocate is used to manipulate the allocated disk space for a file, either to deallocate or preallocate it. For filesystems which support the fallocate system call, preallocation is done quickly by allocating blocks and marking them as uninitialized, requiring no IO to the data blocks. This is much faster than creating a file by filling it with zeroes.
The exit code returned by fallocate is 0 on success and 1 on failure.
OPTIONS
The length
and
offset
arguments may
be followed by the multiplicative suffixes KiB (=1024), MiB
(=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB, and YiB
(the iB is optional, e.g., K has the same meaning as
KiB) or the suffixes KB (=1000), MB (=1000*1000), and so on
for GB, TB, PB, EB, ZB, and YB.
The options −−collapse−range
,
−−dig−holes
,
−−punch−hole
,
and −−zero−range
are mutually
exclusive.
−c, −−collapse−range
-
Removes a byte range from a file, without leaving a hole. The byte range to be collapsed starts at
offset
and continues forlength
bytes. At the completion of the operation, the contents of the file starting at the locationoffset
+length
will be appended at the locationoffset
, and the file will belength
bytes smaller. The option−−keep−size
may not be specified for the collapse-range operation.Available since Linux 3.15 for ext4 (only for extent-based files) and XFS.
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, the operation will fail with the error EINVAL if this requirement is violated.
−d, −−dig−holes
-
Detect and dig holes. This makes the file sparse in-place, without using extra disk space. The minimum size of the hole depends on filesystem I/O block size (usually 4096 bytes). Also, when using this option,
−−keep−size
is implied. If no range is specified by−−offset
and−−length
, then the entire file is analyzed for holes.You can think of this option as doing a cp −−sparse and then renaming the destination file to the original, without the need for extra disk space.
See
−−punch−hole
for a list of supported filesystems. −i, −−insert−range
-
Insert a hole of
length
bytes fromoffset
, shifting existing data. −l, −−length
length
-
Specifies the length of the range, in bytes.
−n, −−keep−size
-
Do not modify the apparent length of the file. This may effectively allocate blocks past EOF, which can be removed with a truncate.
−o, −−offset
offset
-
Specifies the beginning offset of the range, in bytes.
−p, −−punch−hole
-
Deallocates space (i.e., creates a hole) in the byte range starting at
offset
and continuing forlength
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. This option may not be specified at the same time as the−−zero−range
option. Also, when using this option,−−keep−size
is implied.Supported for XFS (since Linux 2.6.38), ext4 (since Linux 3.0), Btrfs (since Linux 3.7) and tmpfs (since Linux 3.5).
−v, −−verbose
-
Enable verbose mode.
−x, −−posix
-
Enable POSIX operation mode. In that mode allocation operation always completes, but it may take longer time when fast allocation is not supported by the underlying filesystem.
−z, −−zero−range
-
Zeroes space in the byte range starting at
offset
and continuing forlength
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.
Option
−−keep−size
can be specified to prevent file length modification.Available since Linux 3.14 for ext4 (only for extent-based files) and XFS.
−V, −−version
-
Display version information and exit.
−h, −−help
-
Display help text and exit.
AVAILABILITY
The fallocate command is part of the util-linux package and is available from Linux Kernel Archive
|
Section (2) fallocate
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_zsingle_quotesz_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_zsingle_quotesz_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
isFALLOC_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, orlen
was less than or equal to 0. - EINVAL
-
mode
isFALLOC_FL_COLLAPSE_RANGE
and the range specified byoffset
pluslen
reaches or passes the end of the file. - EINVAL
-
mode
isFALLOC_FL_INSERT_RANGE
and the range specified byoffset
reaches or passes the end of the file. - EINVAL
-
mode
isFALLOC_FL_COLLAPSE_RANGE
orFALLOC_FL_INSERT_RANGE
, but eitheroffset
orlen
is not a multiple of the filesystem block size. - EINVAL
-
mode
contains one ofFALLOC_FL_COLLAPSE_RANGE
orFALLOC_FL_INSERT_RANGE
and also other flags; no other flags are permitted withFALLOC_FL_COLLAPSE_RANGE
orFALLOC_FL_INSERT_RANGE
. - EINVAL
-
mode
isFALLOC_FL_COLLAPSE_RANGE
orFALLOC_FL_ZERO_RANGE
orFALLOC_FL_INSERT_RANGE
, but the file referred to byfd
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. (Iffd
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 themode
is not supported by the filesystem containing the file referred to byfd
. - EPERM
-
The file referred to by
fd
is marked immutable (see chattr(1)). - EPERM
-
mode
specifiesFALLOC_FL_PUNCH_HOLE
orFALLOC_FL_COLLAPSE_RANGE
orFALLOC_FL_INSERT_RANGE
and the file referred to byfd
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
specifiesFALLOC_FL_COLLAPSE_RANGE
orFALLOC_FL_INSERT_RANGE
, but the file referred to byfd
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.
COLOPHON
This page is part of release 5.04 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/.
Copyright (c) 2007 Silicon Graphics, Inc. All Rights Reserved Written by Dave Chinner <dgcsgi.com> %%%LICENSE_START(GPLv2_ONELINE) May be distributed as per GNU General Public License version 2. %%%LICENSE_END 2011-09-19: Added FALLOC_FL_PUNCH_HOLE 2011-09-19: Substantial restructuring of the page |