linux-kernel - Re: relatime: update once per day patches (was: ext3 IO latency measurements)

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Date:	Thu, 26 Mar 2009 12:30:26 -0400
From:	Theodore Tso <tytso@....edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Frans Pop <elendil@...net.nl>, mingo@...e.hu, jack@...e.cz,
	alan@...rguk.ukuu.org.uk, arjan@...radead.org,
	a.p.zijlstra@...llo.nl, npiggin@...e.de, jens.axboe@...cle.com,
	drees76@...il.com, jesper@...gh.cc, linux-kernel@...r.kernel.org,
	oleg@...hat.com, roland@...hat.com
Subject: Re: relatime: update once per day patches (was: ext3 IO latency
	measurements)

On Thu, Mar 26, 2009 at 09:14:28AM -0700, Linus Torvalds wrote:
> I generally agree witht he "leave policy to user space" people, but this 
> is an area where (a) user space has shown itself to not get it right (ie 
> people don't do even the existing relatime because distros don't) and (b) 
> what's the alternative?

I thought at least some distro's were adding relatime by default; I
could be wrong, but I thought Ubuntu was doing this.  Personally, I
actually think that if we're going to give up on POSIX, I'll go all
the way to noatime since it helps even more.

I've always thought the right approach would be to have a "atime
dirty" flag, and update atime, but never flush it out to disk unless
(a) we're about to unmount the disk, or (b) we need to update some
other inode in the same inode table block, or (c) we have memory
pressure and we're trying to evict the inode from the inode cache.
That way we get full POSIX compliance, without taking the I/O hit of
atime updates.  The atime updates get lost if we crash, but that's
allowed by POSIX, and most people don't care about losing atime
updates after a crash.

Since it's fully backwards (and POSIX) compatible, there would no
question about enabling it by default.

						- Ted
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