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

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Date:	Thu, 26 Mar 2009 18:12:04 +0100
From:	Frans Pop <elendil@...net.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, mingo@...e.hu,
	tytso@....edu, 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 Thursday 26 March 2009, Andrew Morton wrote:
> On Thu, 26 Mar 2009 09:14:28 -0700 (PDT) Linus Torvalds 
<torvalds@...ux-foundation.org> 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 (and others) pointed out that it would be better to implement
> > > this as a mount option.  That suggestion was met with varying
> > > sillinesses and that is where things stand.
> >
> > I'd suggest first just doing the 24 hour thing, and then, IF user
> > space actually ever gets its act together, and people care, and they
> > _ask_ for a mount option, that's when it's worth doing.
>
> We wouldn't normally just enable the new feature by default because it
> changes kernel behaviour.  Userspace needs to be changed in some manner
> to opt-in.  One way it's `mount -o remount', the other way it's a poke
> in /proc.

What change are you talking about here exactly? The "change relatime to 
have a 24 hour safeguard" of Matthes's first patch or the "enable 
relatime by default" options in the second patch?

For the first I don't think it's that big a deal as it is a change that 
makes the behavior of relatime safer and not riskier. Also, it's 
something people have argued should have been part of the initial 
functionality of relatime (it was part of the discussion back then), and 
finally for a lot of users it's already current functionality as major 
distros already do include the patch.

For the second, I can see your point and can understand reservations to 
make enabling relatime a kernel config option.

Speaking exclusively for myself, I would be happy enough if only the first 
of Matthew's patches would get accepted.

Cheers,
FJP
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