linux-kernel - Re: [GIT PULL] Ext3 latency fixes

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Date:	Sat, 4 Apr 2009 10:44:27 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Theodore Tso <tytso@....edu>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [GIT PULL] Ext3 latency fixes

On Sat, 4 Apr 2009, Jens Axboe wrote:
> > 
> > I'm sorry, but that fsync thing _is_ a real-world case, and it's the one 
> > that a hell of a lot more people care about than some idiotic sqlite 
> > throughput issue.
> 
> sqlite is just one case, I'm sure there are others. My point is that we
> should make sure that we don't regress on the throughput side. It's a
> trade off, we don't want throughput to fall through the floor either.

Jens, we _have_ regressed on the latency side. Everybody agrees. 

Also, I may be odd, but I really do think latency is more important than 
throughput. When my disk has latencies in the sub-milliseconds, I simply 
do not think it is _acceptable_ to have hickups that affect my workload in 
human-visible terms. 

You say sqlite might regress by 4-5x. But Ted's numbers improve latencies 
by mor than that. I haven't re-created them yet myself (still reading 
email), but the point is, 4-5x may sound bad to you, but turn it around: 
the current latency situation is _really_ bad. If we can fix it, we 
definitely should.

> > Quite frankly, the fact that I can see _seconds_ of latencies with a 
> > really good SSD is not acceptable. The fact that it is by design is even 
> > less so.
> 
> Agree, multi-second latencies is not acceptable.

I can literally send you strace output from my MUA, where it pauses for 
ten seconds after it has written about 5kB (that's _kilobytes_) of data 
and does a 'fsync'.

That's the load that Ted worked on and has a solution for.

		Linus
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