linux-kernel - Re: [PATCH 00/23] per device dirty throttling -v8

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Date:	Sat, 4 Aug 2007 10:39:56 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	miklos@...redi.hu, akpm@...ux-foundation.org, neilb@...e.de,
	dgc@....com, tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com,
	richard@....demon.co.uk, david@...g.hm
Subject: Re: [PATCH 00/23] per device dirty throttling -v8

On Sat, 4 Aug 2007, Ingo Molnar wrote:
> 
> yeah, it's really ugly. But otherwise i've got no real complaint about 
> ext3 - with the obligatory qualification that "noatime,nodiratime" in 
> /etc/fstab is a must.

I agree, we really should do something about atime.

But the fsync thing is a real issue. It literally makes ext3 almost 
unusable from a latency standpoint on many loads. I have a fast disk, and 
don't actually tend to have all that much going on normally, and it still 
hurts occasionally. 

One of the most common (and *best*) reasons for using fsync is for the 
mail spool. So anybody that uses local email will actually be doing a lot 
of fsync, and while you could try to thread the interfaces, I don't think 
a lot of mailers do.

So fsync ends up being a latency issue for something that a lot of people 
actually see, and something that you actually end up working with and you 
notice the latencies very clearly. Your editor auto-save feature is 
another good example of that exact same thing: the fsync actually is there 
for a very good reason, even if you apparently decided that you'd rather 
disable it.

But yeah, "noatime,data=writeback" will quite likely be *quite* noticeable 
(with different effects for different loads), but almost nobody actually 
runs that way.

I ended up using O_NOATIME for the individual object "open()" calls inside 
git, and it was an absolutely huge time-saver for the case of not having 
"noatime" in the mount options. Certainly more than your estimated 10% 
under some loads.

The "relatime" thing that David mentioned might well be very useful, but 
it's probably even less used than "noatime" is. And sadly, I don't really 
see that changing (unless we were to actually change the defaults inside 
the kernel).

			Linus
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