directio

README

Linux Direct IO Writer

Direct IO writer using O_DIRECT

[!WARNING] /tmp/ in modern systems doesn't supoprt Direct I/O, for tests use /var/tmp instead.

Example:

package main

import (
    "io"
    "log"
    "net/http"
    "os"
    "syscall"

    "github.com/brk0v/directio"
)

func main() {
    // Open file with O_DIRECT
    flags := os.O_WRONLY | os.O_EXCL | os.O_CREATE | syscall.O_DIRECT
    f, err := os.OpenFile("/var/tmp/mini.iso", flags, 0644)
    if err != nil {
        log.Fatal(err)
    }
    defer f.Close()

    // Use directio writer
    dio, err := directio.New(f)
    if err != nil {
        log.Fatal(err)
    }
    defer dio.Flush()

    // Downloading iso image
    resp, err := http.Get("http://archive.ubuntu.com/ubuntu/dists/bionic/main/installer-amd64/current/images/netboot/mini.iso")
    if err != nil {
        log.Fatal(err)
    }
    defer resp.Body.Close()

    // Write the body to file
    _, err = io.Copy(dio, resp.Body)
}

Check that dio bypass linux pagecache using vmtouch:

$ vmtouch /var/tmp/mini.iso
           Files: 1
     Directories: 0
  Resident Pages: 1/16384  4K/64M  0.0061%
         Elapsed: 0.000356 seconds

or using my https://github.com/brk0v/cpager to check per cgroup pagecache usage:

$ sudo ~/go/bin/cpager /var/tmp/mini.iso
         Files: 1
   Directories: 0
Resident Pages: 1/16385 4K/64M 0.0%

 cgmem inode    percent       pages        path
           -     100.0%       16384        not charged
        2187       0.0%           1        /sys/fs/cgroup/memory/user.slice/user-1000.slice/session-3.scope

Documentation

Index

• Constants
• Variables
• func DIOMemAlign(path string) (uint32, error)
• type DirectIO
  • func New(f *os.File) (*DirectIO, error)
  • func NewSize(f *os.File, size int) (*DirectIO, error)
  • func (d *DirectIO) Available() int
  • func (d *DirectIO) Buffered() int
  • func (d *DirectIO) Close() error
  • func (d *DirectIO) Flush() error
  • func (d *DirectIO) Write(p []byte) (nn int, err error)

Constants

const (
	O_DIRECT = syscall.O_DIRECT
)

Variables

var ErrFSNoDIOSupport = errors.New("filesystem does not expose Direct I/O alignment")

var ErrNotSetDirectIO = errors.New("O_DIRECT flag is absent")

Functions

func DIOMemAlign

func DIOMemAlign(path string) (uint32, error)

Types

type DirectIO

type DirectIO struct {
	// contains filtered or unexported fields
}

DirectIO bypasses page cache.

func New

func New(f *os.File) (*DirectIO, error)

New returns a new DirectIO writer with default buffer size.

func NewSize

func NewSize(f *os.File, size int) (*DirectIO, error)

NewSize returns a new DirectIO writer.

func (*DirectIO) Available

func (d *DirectIO) Available() int

Available returns how many bytes are unused in the buffer.

func (*DirectIO) Buffered

func (d *DirectIO) Buffered() int

Buffered returns the number of bytes that have been written into the current buffer.

func (*DirectIO) Close

func (d *DirectIO) Close() error

func (*DirectIO) Flush

func (d *DirectIO) Flush() error

Flush writes buffered data to the underlying file.

func (*DirectIO) Write

func (d *DirectIO) Write(p []byte) (nn int, err error)

Write writes the contents of p into the buffer. It returns the number of bytes written. If nn < len(p), it also returns an error explaining why the write is short.