linux-ext4 - Re: Ext4 slow on links

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Message-ID: <4FE1D91B.8020707@redhat.com>
Date:	Wed, 20 Jun 2012 09:07:23 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Norbert Preining <preining@...ic.at>
CC:	"Ted Ts'o" <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Ext4 slow on links

On 6/20/12 12:18 AM, Norbert Preining wrote:
> Hi Eric,
> 
> On Di, 19 Jun 2012, Eric Sandeen wrote:
>> blktrace of the actions would show you something interesting as well.
> 
> I tried to understand the output, but didn't get any information
> that tells me something.
> 
> I rebooted into single user mode, started blktrace on sda, then run
> time ls -l /..../dir/with/links/ >/dev/null, stopped the blktrace.
> 
> Then I run blkparse and btt etc to generate a variety of data.

...

> 
> I don't know if that shows anything of interest, but if you need more,
> and want to waste a bit of time looking at the data, I have uploaded
> everything created into
> 	http://www.logic.at/people/preining/BlkParse.tar.gz

Here are the overall stats:

Total (sda):
 Reads Queued:       8,864,   35,456KiB	 Writes Queued:          90,    7,980KiB
 Read Dispatches:    8,864,   35,456KiB	 Write Dispatches:       49,    7,980KiB
 Reads Requeued:         0		 Writes Requeued:         0
 Reads Completed:    8,864,   35,456KiB	 Writes Completed:       59,    7,980KiB
 Read Merges:            0,        0KiB	 Write Merges:           41,      164KiB
 IO unplugs:            81        	 Timer unplugs:           0

so almost all reads, and no read merges; almost 35 megabytes read and every
one was a small 4k IO.

It's doing about 120 seeks/second.  I'm a little surprised that there was no read
merging...

Let me think about this. :)

-Eric
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