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How to track down abysmal performance ata - raid1 - crypto - vg/lv - xfs

To: linux-raid@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
Subject: How to track down abysmal performance ata - raid1 - crypto - vg/lv - xfs
From: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Date: Wed, 4 Aug 2010 09:35:46 +0200
Mail-followup-to: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>, linux-raid@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
User-agent: Mutt/1.5.20 (2009-06-14)
Hey,

on a production system I run kernel 2.6.35 and

        XFS     (rw,relatime,nobarrier)

on a

        lvdevice of a vgroup 

consisting of five

        dm-crypt devices (cryptsetup -c aes-lrw-benbi -s 384 create)

, each of which runs on a

        md-raid1 device (mdadm --create --level=raid1 --raid-devices=2)

on two

        750 GB ATA devices.


The read performance is abysmal. The ata devices can be ruled out, as hdparm
resulted in acceptable performance:
> Timing cached reads:   9444 MB in  2.00 seconds = 4733.59 MB/sec
> Timing buffered disk reads:  298 MB in  3.02 seconds =  98.73 MB/sec

How can I best track down the cause of the performance problem, 
a) without rebooting too often, and
b) without breaking up the setup specified above (production system)?

Any ideas? perf(1)? iostat(1)?

Thanks & best,

        Dominik

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