Os File Direct Io

In this tutorial, you'll learn about How to Fix Direct I/O Errors. We cover key concepts, practical examples, and best practices.

Fix direct i/o errors when kernel page cache caching large files wastes memory.

Quick Fix

Wrong

with open('large.bin','rb') as f:
    f.read()  # goes through page cache!

Large file pollutes page cache. Evicts useful cached data. Doubles memory usage.

Right

import os
# Use O_DIRECT to bypass page cache:
fd=os.open('large.bin', os.O_RDONLY | os.O_DIRECT)
# Must align buffer (typically 512 or 4096):
import mmap
# Or use posix_fadvise to bypass cache:
import ctypes
libc=ctypes.CDLL('libc.so.6')
POSIX_FADV_DONTNEED=4
with open('large.bin','rb') as f:
    libc.posix_fadvise(f.fileno(), 0, 0, POSIX_FADV_DONTNEED)
    data=f.read(4096)

Data read directly from disk. Page cache not polluted. O_DIRECT requires aligned buffers.

Prevention

Use O_DIRECT for large sequential reads/writes. Use fadvise to hint cache policy.

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