fallocate (1)

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

fallocate -d [-o offset] [-l length] filename

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.

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.

-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.

-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).

-d, --dig-holes

Detect and dig holes. Makes the file sparse in-place, without using extra disk space. The minimal 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 all file is analyzed for holes.

You can think of this as doing a "cp --sparse" and renaming the dest file as the original, without the need for extra disk space.

See --punch-hole for list of the supported filesystems.

-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.

-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.

-o, --offset offset

Specifies the beginning offset of the range, in bytes.

-l, --length length

Specifies the length of the range, in bytes.

-h, --help

Display help text and exit.

-v, --verbose

Enable verbose mode.

-V, --version

Display version information and exit.

Eric Sandeen

Karel Zak

fallocate(2), posix_fallocate(3), truncate(1)

The fallocate command is part of the util-linux package and is available from

Linux Kernel Archive