linux-kernel - Re: Linux 2.6.29

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Date:	Thu, 26 Mar 2009 18:25:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Theodore Tso <tytso@....edu>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Thu, 26 Mar 2009 18:03:15 -0700 (PDT) Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Thu, 26 Mar 2009, Andrew Morton wrote:
> > 
> > userspace can get closer than the kernel can.
> 
> Andrew, that's SIMPLY NOT TRUE.
> 
> You state that without any amount of data to back it up, as if it was some 
> kind of truism. It's not.

I've seen you repeatedly fiddle the in-kernel defaults based on
in-field experience.  That could just as easily have been done in
initscripts by distros, and much more effectively because it doesn't
need a new kernel.  That's data.

The fact that this hasn't even been _attempted_ (afaik) is deplorable.

Why does everyone just sit around waiting for the kernel to put a new
value into two magic numbers which userspace scripts could have set?

My /etc/rc.local has been tweaking dirty_ratio, dirty_background_ratio
and swappiness for many years.  I guess I'm just incredibly advanced.

> Everybody accepts that if you've written a 20MB file and then call 
> "fsync()" on it, it's going to take a while. But when you've written a 2kB 
> file, and "fsync()" takes 20 seconds, because somebody else is just 
> writing normally, _that_ is a bug. And it is actually almost totally 
> unrelated to the whole 'dirty_limit' thing.
> 
> At least it _should_ be. 

That's different.  It's inherent JBD/ext3-ordered brain damage. 
Unfixable without turning the fs into something which just isn't jbd/ext3
any more.  data=writeback is a workaround, with the obvious integrity
issues.

The JBD journal is a massive designed-in contention point.  It's why
for several years I've been telling anyone who will listen that we need
a new fs.  Hopefully our response to all these problems will soon be
"did you try btrfs?".

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