ACP: `File::preallocate`

Proposal

Problem statement

When writing large files to disk, if you know the file size in advance, you should be able to preallocate the space for the file. This provides a substantial performance improvement, and also helps the filesystem avoid fragmentation that might otherwise cause multiple files written in parallel to get interleaved.

Motivating examples or use cases

Any application or library that writes large files whose sizes it knows in advance.

Solution sketch

impl File {
    pub fn preallocate(&self, len: u64) -> io::Result<()>;
}

Alternatives

There are various Linux-specific additional modes/operations that fallocate provides, which this API is not attempting to support. Those might make sense as future separate methods on an extension trait.

Links and related work

https://www.man7.org/linux/man-pages/man2/fallocate.2.html

https://docs.rs/fs4/latest/fs4/trait.FileExt.html#tymethod.allocate

https://docs.rs/rustix/latest/rustix/fs/fn.fallocate.html

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start by reading the ACP proposal and comparing the linked fallocate, fs4, and rustix references. The issue is awaiting libs-api review, so completion is not defined until the team decides whether the problem and proposed File::preallocate API belong in the standard library.