.\" 2011-09-19: Added FALLOC_FL_PUNCH_HOLE
.\" 2011-09-19: Substantial restructuring of the page
.\"
-.TH FALLOCATE 2 2015-04-19 "Linux" "Linux Programmer's Manual"
+.TH FALLOCATE 2 2018-04-30 "Linux" "Linux Programmer's Manual"
.SH NAME
fallocate \- manipulate file space
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
.B #include <fcntl.h>
-
+.PP
.BI "int fallocate(int " fd ", int " mode ", off_t " offset \
", off_t " len ");
.fi
For the portable, POSIX.1-specified method of ensuring that space
is allocated for a file, see
.BR posix_fallocate (3).
-
+.PP
.BR fallocate ()
allows the caller to directly manipulate the allocated disk space
for the file referred to by
and continuing for
.I len
bytes.
-
+.PP
The
.I mode
argument determines the operation to be performed on the given range.
.BR posix_fallocate (3)
library function,
and is intended as a method of optimally implementing that function.
-
+.PP
After a successful call, subsequent writes into the range specified by
.IR offset
and
.IR len
are guaranteed not to fail because of lack of disk space.
-
+.PP
If the
.B FALLOC_FL_KEEP_SIZE
flag is specified in
Preallocating zeroed blocks beyond the end of the file in this manner
is useful for optimizing append workloads.
.PP
+If the
+.B FALLOC_FL_UNSHARE
+flag is specified in
+.IR 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.
+.PP
Because allocation is done in block size chunks,
.BR fallocate ()
may allocate a larger range of disk space than was specified.
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.
-
+subsequent reads from this range will return zeros.
+.PP
The
.BR FALLOC_FL_PUNCH_HOLE
flag must be ORed with
(as reported by
.BR stat (2))
does not change.
-
+.PP
Not all filesystems support
.BR FALLOC_FL_PUNCH_HOLE ;
if a filesystem doesn't support the operation, an error is returned.
.IP *
Btrfs (since Linux 3.7)
.IP *
-tmpfs (since Linux 3.5)
+.BR tmpfs (5)
+(since Linux 3.5)"
.\" commit 83e4fa9c16e4af7122e31be3eca5d57881d236fe
.SS Collapsing file space
.\" commit 00f5e61998dd17f5375d9dfc01331f104b83f841
and the file will be
.I len
bytes smaller.
-
+.PP
A filesystem may place limitations on the granularity of the operation,
in order to ensure efficient implementation.
Typically,
which varies according to the filesystem type and configuration.
If a filesystem has such a requirement,
.BR fallocate ()
-will fail with the error
+fails with the error
.BR EINVAL
if this requirement is violated.
-
+.PP
If the region specified by
.I offset
plus
instead, use
.BR ftruncate (2)
to truncate a file.
-
+.PP
No other flags may be specified in
.IR mode
in conjunction with
.BR FALLOC_FL_COLLAPSE_RANGE .
-
+.PP
As at Linux 3.15,
.B FALLOC_FL_COLLAPSE_RANGE
is supported by
.\" commit 409332b65d3ed8cfa7a8030f1e9d52f372219642
in
.I mode
-zeroes space in the byte range starting at
+zeros space in the byte range starting at
.I offset
and continuing for
.I len
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.
-
+reads from this range will return zeros.
+.PP
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.
-
+.PP
If the
.B FALLOC_FL_KEEP_SIZE
flag is additionally specified in
This behavior is the same as when preallocating space with
.B FALLOC_FL_KEEP_SIZE
specified.
-
+.PP
Not all filesystems support
.BR FALLOC_FL_ZERO_RANGE ;
if a filesystem doesn't support the operation, an error is returned.
.IP *
SMB3 (since Linux 3.17)
.\" commit 30175628bf7f521e9ee31ac98fa6d6fe7441a556
+.IP *
+Btrfs (since Linux 4.16)
+.\" commit f27451f229966874a8793995b8e6b74326d125df
.SS Increasing file space
Specifying the
.BR FALLOC_FL_INSERT_RANGE
Inserting a hole inside a file increases the file size by
.I len
bytes.
-
+.PP
This mode has the same limitations as
.BR FALLOC_FL_COLLAPSE_RANGE
regarding the granularity of the operation.
If the granularity requirements are not met,
.BR fallocate ()
-will fail with the error
+fails with the error
.BR EINVAL.
If the
.I offset
For such operations (i.e., inserting a hole at the end of file),
.BR ftruncate (2)
should be used.
-
+.PP
No other flags may be specified in
.IR mode
in conjunction with
.BR FALLOC_FL_INSERT_RANGE .
-
+.PP
.B FALLOC_FL_INSERT_RANGE
-requires filesystem support, which is currently provided only by
-XFS (since Linux 4.1).
+requires filesystem support.
+Filesystems that support this operation include
+XFS (since Linux 4.1)
.\" commit a904b1ca5751faf5ece8600e18cd3b674afcca1b
+and ext4 (since Linux 4.2).
+.\" commit 331573febb6a224bc50322e3670da326cb7f4cfc
+.\" f2fs also has support since Linux 4.2
+.\" commit f62185d0e283e9d311e3ac1020f159d95f0aab39
.SH RETURN VALUE
On success,
.BR fallocate ()
and the current file size+\fIlen\fP exceeds the maximum file size.
.TP
.B EINTR
-A signal was caught during execution.
+A signal was caught during execution; see
+.BR signal (7).
.TP
.B EINVAL
.I offset
but the file referred to by
.I fd
is not a regular file.
-.\" There was a inconsistency in 3.15-rc1, that should be resolved so that all
+.\" There was an inconsistency in 3.15-rc1, that should be resolved so that all
.\" filesystems use this error for this case. (Tytso says ex4 will change.)
.\" http://thread.gmane.org/gmane.comp.file-systems.xfs.general/60485/focus=5521
.\" From: Michael Kerrisk (man-pages <mtk.manpages@...>
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