07-09-2014
You use direct IO on Linux by calling open() with the O_DIRECT flag. The buffers used for read()/write() calls might need to have a specific alignment, most likely page-aligned, see the man pages for valloc() and memalign(). Also, both the number of bytes transferred by each read()/write() call and the offset read from/written to might also need to be an exact multiple of a fundamental filesystem or hardware block size. That could cause problems reading/writing the last block of a file, depending on the version of Linux you have, the file system you're using, and even maybe your hardware.
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LEARN ABOUT DEBIAN
e4defrag
E4DEFRAG(8) System Manager's Manual E4DEFRAG(8)
NAME
e4defrag - online defragmenter for ext4 filesystem
SYNOPSIS
e4defrag [ -c ] [ -v ] target ...
DESCRIPTION
e4defrag reduces fragmentation of extent based file. The file targeted by e4defrag is created on ext4 filesystem made with "-O extent"
option (see mke2fs(8)). The targeted file gets more contiguous blocks and improves the file access speed.
target is a regular file, a directory, or a device that is mounted as ext4 filesystem. If target is a directory, e4defrag reduces fragmen-
tation of all files in it. If target is a device, e4defrag gets the mount point of it and reduces fragmentation of all files in this mount
point.
OPTIONS
-c Get a current fragmentation count and an ideal fragmentation count, and calculate fragmentation score based on them. By seeing this
score, we can determine whether we should execute e4defrag to target. When used with -v option, the current fragmentation count and
the ideal fragmentation count are printed for each file.
Also this option outputs the average data size in one extent. If you see it, you'll find the file has ideal extents or not. Note
that the maximum extent size is 131072KB in ext4 filesystem (if block size is 4KB).
If this option is specified, target is never defragmented.
-v Print error messages and the fragmentation count before and after defrag for each file.
NOTES
e4defrag does not support swap file, files in lost+found directory, and files allocated in indirect blocks. When target is a device or a
mount point, e4defrag doesn't defragment files in mount point of other device.
Non-privileged users can execute e4defrag to their own file, but the score is not printed if -c option is specified. Therefore, it is
desirable to be executed by root user.
AUTHOR
Written by Akira Fujita <a-fujita@rs.jp.nec.com> and Takashi Sato <t-sato@yk.jp.nec.com>.
SEE ALSO
mke2fs(8), mount(8).
e4defrag version 2.0 May 2009 E4DEFRAG(8)