linux-kernel - Re: Linux 2.6.29

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Date:	Sun, 29 Mar 2009 21:55:59 -0600
From:	"Trenton D. Adams" <trenton.d.adams@...il.com>
To:	Theodore Tso <tytso@....edu>,
	Trenton Adams <trenton.d.adams@...il.com>,
	Mark Lord <lkml@....ca>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Jeff Garzik <jeff@...zik.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 Sun, Mar 29, 2009 at 9:28 PM, Theodore Tso <tytso@....edu> wrote:
> On Sun, Mar 29, 2009 at 07:29:09PM -0600, Trenton Adams wrote:
>> I am slightly confused by the "data=ordered" thing that everyone is
>> mentioning of late.  In theory, it made sense to me before I tried it.
>>  I switched to mounting my ext3 as ext4, and I'm still seeing
>> seriously delayed fsyncs.  Theodore, I used a modified version of your
>> fsync-tester.c to bench 1M writes, while doing a dd, and I'm still
>> getting *almost* as bad of "fsync" performance as I was on ext3.  On
>> ext3, the fsync would usually not finish until the dd was complete.
>
> How much memory do you have?  On my 4gig X61 laptop, using a 5400 rpm
> laptop drive, I see typical times of 1 to 1.5 seconds, with a few
> outliers at 4-5 seconds.  With ext3, the fsync times immediately
> jumped up to 6-8 seconds, with the outliers in the 13-15 second range.

2G, and I believe 5400rpm

>
> (This is with a filesystem formated as ext3, and mounted as either
> ext3 or ext4; if the filesystem is formatted using "mke2fs -t ext4",
> what you see is a very smooth 1.2-1.5 seconds fsync latency, indirect
> blocks for very big files end up being quite inefficient.)

Oh.  I thought I had read somewhere that mounting ext4 over ext3 would
solve the problem.  Not sure where I read that now.  Sorry for wasting
your time.

>
> So I'm seeing a definite difference --- but also please remember that
> "dd if=/dev/zero of=bigzero.img" really is an unfair, worst-case
> scenario, since you are dirtying memory as fast as your CPU will dirty
> pages.  Normally, even if you are running distcc, the rate at which
> you can dirty pages will be throttled at your local network speed.

Yes, I realize that.  When trying to find performance problems I try
to be as *unfair* as possible. :D

Thanks Ted.
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