fallocate

fallocate exposes an interface to fallocate(2), posix_fallocate(3) and posix_fadvise(3).

Under Mac OS X the fallocate() method will use the apple equivalent of fallocate(2). Note that this might not be exactly the same.

When using the wrapper functions around the fallocate(2) call the arguments given to the function are slightly different then the c call.

This module has the arguments like:

fallocate(fd, offset, length, mode=0)

While in C the function looks like:

fallocate(fd, mode, offset, length)

The main reason for this is that the mode argument tends not to be used much and thus having the default as a keyword argument is much easier then having to specify 0 everytime.

Windows is not supported at this point, as there is no efficient way of ensuring zero-initialization. See StackOverflow for details.

Usage

Funcation: fallocate(fd, offset, len, [mode=0])

Calls fallocate() on the file object or file descriptor. This 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.

mode is only available in Linux.

It should always be 0 unless one of the following possible flags are specified

FALLOC_FL_KEEP_SIZE     - do not grow file, default is extend size
FALLOC_FL_PUNCH_HOLE    - punches a hole in file, de-allocates range
FALLOC_FL_COLLAPSE_SIZE - remove a range of a file without leaving a hole

Note: FALLOC_FL_COLLAPSE_SIZE was introduced in Linux kernel v3.15 and is only available on certain filesystems (e.g. ext4, xfs, etc). In order to get access to it, you must build and run on a kernel and filesystem that both support it.

Example:

from fallocate import fallocate

# preallocate using fallocate a 1kb file
with open("/tmp/test.file", "w+b") as f:
    fallocate(f, 0, 1024)