base/src/sys/linux/write_zeroes.rs

// Copyright 2018 The ChromiumOS Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use std::cmp::min;
use std::fs::File;
use std::io;
use std::os::unix::fs::FileExt;

use super::discard_block;
use super::fallocate;
use super::is_block_file;
use super::FallocateMode;

pub(crate) fn file_punch_hole(file: &File, offset: u64, length: u64) -> io::Result<()> {
    if is_block_file(file)? {
        Ok(discard_block(file, offset, length)?)
    } else {
        fallocate(file, FallocateMode::PunchHole, offset, length)
            .map_err(|e| io::Error::from_raw_os_error(e.errno()))
    }
}

pub(crate) fn file_write_zeroes_at(file: &File, offset: u64, length: usize) -> io::Result<usize> {
    // Try to use fallocate() first.
    if fallocate(file, FallocateMode::ZeroRange, offset, length as u64).is_ok() {
        return Ok(length);
    }

    // fall back to write()
    // fallocate() failed; fall back to writing a buffer of zeroes
    // until we have written up to length.
    let buf_size = min(length, 0x10000);
    let buf = vec![0u8; buf_size];
    let mut nwritten: usize = 0;
    while nwritten < length {
        let remaining = length - nwritten;
        let write_size = min(remaining, buf_size);
        nwritten += file.write_at(&buf[0..write_size], offset + nwritten as u64)?;
    }
    Ok(length)
}