fallocate Man page

FALLOCATE(1) User Commands FALLOCATE(1)

NAME

fallocate – preallocate or deallocate space to a file

SYNOPSIS

fallocate [-c] [-n] [-p] [-z] [-o offset] -l length filename

fallocate -d [-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
zeros.

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 for
length bytes. At the completion of the operation, the contents
of the file starting at the location offset+length will be
appended at the location offset, and the file will be length
bytes smaller. The option –keep-size may not be specified for
colapse range operation.

Available since Linux 3.15 for ext4 (only for extent-based
files) and XFS.

-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.

-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 effec‐
tively 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 for length 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.

-z, –zero-range
Zeroes space in the byte range starting at offset and continuing
for length bytes. Within the specified range, blocks are preal‐
located 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 modi‐
fication.

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.

AUTHORS
Eric Sandeen ⟨sandeen@redhat.com⟩
Karel Zak ⟨kzak@redhat.com⟩

SEE ALSO

fallocate, posix_fallocate, truncate

AVAILABILITY
The fallocate command is part of the util-linux package and is avail‐
able from Linux Kernel Archive ⟨ftp://ftp.kernel.org/pub/linux/utils
/util-linux/⟩.

util-linux September 2011 FALLOCATE(1)

Ils en parlent aussi

dd vs vs truncate – making big files quick – kossboss