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Date:	Wed, 25 Jul 2012 20:35:35 -0700
From:	Marc MERLIN <marc@...lins.org>
To:	Ted Ts'o <tytso@....edu>
Cc:	Lukáš Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org, axboe@...nel.dk,
	Milan Broz <mbroz@...hat.com>
Subject: Re: du -s src is a lot slower on SSD than spinning disk in the same laptop

On Wed, Jul 25, 2012 at 11:32:23PM -0400, Ted Ts'o wrote:
> On Wed, Jul 25, 2012 at 08:40:28PM +0200, Lukáš Czerner wrote:
> > > First, I had he problem with btrfs (details below), and then I noticed that
> > > while ext4 is actually twice as fast as btrfs, it's still a lot slower at
> > > stat on my fast Samsung 830 512G SSD than my 1TB laptop hard drive (both
> > > drives being in the same thinkpad T530 with 3.4.4 kernel).
> > > 
> > > How can things be so slow? 12-13 seconds to scan 15K inodes on a freshly
> > > made filesystem (and 22 secs with btrfs, so at least ext4 wins). The same
> > > thing on my spinning drive takes fewer than 4 seconds, and SSD should be
> > > several times faster than that.
> > > SSD throughput was measured over 400MB/s on the raw device and 268MB/s
> > > through the filesystem:
> 
> Was this an identical file system image on HDD and SSD?
> 
> The obvious thing to do is to get a blktrace of du -sh w/ a cold cache
> for both the HDD and SSD.  Regardless of whether it's something we can
> address at the fs level, or in the block device layer, the blktrace
> should make it really clear what is going on.

I'm not very familiar with blktrace, but here is one run I did when I was
trying to debug why dd of my dmcrypted FS was running at 27MB/s instead of
270MB/s.
http://marc.merlins.org/tmp/blktrace.txt
(dd shows up as 'bash' for some reason).

If that format isn't good, can you tell me which blktrace command lines I
should use to get you the exact output you'd like?

Thanks,
Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/  
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