linux-kernel - Re: Slow disks.

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Date:	Mon, 20 Dec 2010 19:06:30 +0100
From:	Bruno Prémont <bonbons@...ux-vserver.org>
To:	Rogier Wolff <R.E.Wolff@...Wizard.nl>
Cc:	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org
Subject: Re: Slow disks.

Hi,

[ccing linux-ide]

Please provide the part of kernel log showing initialization of your
disk controller(s) as well as detection of all the discs.
Verbose lspci output for the disc controller and $(smartctl -i -A $disk)
output might be useful as well.

Did you try the individual discs on a completely different system (e.g.
plain desktop system) and what revision of SATA are both components
supporting?

Bruno


On Mon, 20 December 2010 Rogier Wolff <R.E.Wolff@...Wizard.nl> wrote:
> Hi,
> 
> A friend of mine has a server in a datacenter somewhere. His machine
> is not working properly: most of his disks take 10-100 times longer
> to process each IO request than normal. 
> 
> iostat -kx 10 output: 
> Device: rrqm/s wrqm/s r/s  w/s  rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
> sdd     0.30   0.00   0.40 1.20 2.80  1.10  4.88     0.43  271.50 271.44  43.43
> 
> shows that in this 10 second period, the disk was busy for 4.3 seconds
> and serviced 15-16 requests during that time.
> 
> Normal disks show "svctm" of around 10-20ms. 
> 
> Now you might say: It's his disk that's broken.
> Well no: I don't believe that all four of his disks are broken. 
> (I just showed you output about one disk, but there are 4 disks in there
> all behaving similar, but some are worse than others.)
> 
> Or you might say: It's his controller that's broken. So we thought
> too. We replaced the onboard sata controller with a 4-port sata
> card. Now they are running off the external sata card... Slightly
> better, but not by much.
> 
> Or you might say: it's hardware. But suppose the disk doesn't properly
> transfer the data 9 times out of 10, wouldn't the driver tell us
> SOMETHING in the syslog that things are not fine and dandy? Moreover,
> In the case above, 12kb were transferred in 4.3 seconds. If CRC errors
> were happening, the interface would've been able to transfer over
> 400Mb during that time. So every transfer would need to be retried on
> average 30000 times... Not realistic. If that were the case, we'd
> surely hit a maximum retry limit every now and then?
> 
> 
> These syptoms started when the system was running 2.6.33, but are
> still present now the system has been upgraded to 2.6.36.
> 
> Is there anything you can suggest to get to the root of this problem?
> Could this be a software issue with the driver? Can we enable some
> driver debugging to find out what is wrong?
> 
> Any help will be appreciated. 
> 
> 	Roger.
> 
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