fallocate - Online in the Cloud

PROGRAM:

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 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 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 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.

AUTHORS

Eric Sandeen ⟨[email protected]⟩
Karel Zak ⟨[email protected]⟩

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