linux-kernel - Re: Linux 2.6.29

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Date:	Thu, 2 Apr 2009 09:46:14 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Rees <drees76@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Janne Grunau <j@...nau.net>,
	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>,
	Theodore Tso <tytso@....edu>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Thu, 2 Apr 2009, David Rees wrote:
> 
> MythTV actually uses fdatasync, not fsync (or at least that's what it
> did last time I looked at the source).  Not sure how the behavior of
> fdatasync compares to sync_file_range.

fdatasync() _waits_ for the data to hit the disk.

sync_file_range() just starts writeout.

It _can_ do more - you can also ask for it to wait for previous write-out 
in order to start _new_ writeout, or wait for the result, but you wouldn't 
want to, not for something like this.

sync_file_range() is really a much nicer interface, and is a more extended 
fdatasync() that actually matches what the kernel does internally.

You can think of fdatasync(fd) as a 

	sync_file_range(fd, 0, ~0ull,
		SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE | SYNC_FILE_RANGE_WAIT_AFTER);

and then you see why fdatasync is such a horrible interface.

		Linus
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