High iowait

High iowait

[jassduec]

I have a storage raid attached to my linux server running CentOS 4.3
kernel 2.6.9-39.0.2.ELsmp. My server goes into huge iowait when i try
to read/write lot of data to the storage device. How can i debug
whether the problem is with the storage device or some setting of the
operating system. Is there any known issue with this kernel? How can i
tune my system to reduce iowait time?

TIA

Re: High iowait

[Lothar Braun]

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Hi,

On Tuesday 22 August 2006 04:31, jassduec@gmail.com wrote:
> I have a storage raid attached to my linux server running CentOS 4.3
> kernel 2.6.9-39.0.2.ELsmp. My server goes into huge iowait when i try
> to read/write lot of data to the storage device. How can i debug
> whether the problem is with the storage device or some setting of the
> operating system. Is there any known issue with this kernel? How can i
> tune my system to reduce iowait time?

Why should you want to reduce the iowait time? Iowait, as shown by top, means 
afaik that there are some processes waiting for io to complete  and nothing 
else to do for the processor.
If you're running some CPU eating processes while your I/O operations are 
performed, then the iowait value should go down (this is because your CPU can 
then do more sophisticated work than waiting)

Regards,
Lothar

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Re: High iowait

[jassduec]

Lothar,

Thanks for the reply.

Basically i am not able to achieve the kind of throughput which i
should get from my RAID system. And i think the OS is the culprit
(some bug in the vendor kernel) since for some reason it goes into
huge iowaits preventing the applications to read/write more data. How
can i debug whether the problem is with the OS or with the device?

TIA

On 8/22/06, Lothar Braun <mail@lobraun.de> wrote:
> Hi,
>
> On Tuesday 22 August 2006 04:31, jassduec@gmail.com wrote:
> > I have a storage raid attached to my linux server running CentOS 4.3
> > kernel 2.6.9-39.0.2.ELsmp. My server goes into huge iowait when i try
> > to read/write lot of data to the storage device. How can i debug
> > whether the problem is with the storage device or some setting of the
> > operating system. Is there any known issue with this kernel? How can i
> > tune my system to reduce iowait time?
>
> Why should you want to reduce the iowait time? Iowait, as shown by top,
> means
> afaik that there are some processes waiting for io to complete  and nothing
> else to do for the processor.
> If you're running some CPU eating processes while your I/O operations are
> performed, then the iowait value should go down (this is because your CPU
> can
> then do more sophisticated work than waiting)
>
> Regards,
> Lothar
>
>

Re: High iowait

[Art Wildman]

jassduec@gmail.com wrote:
> Basically i am not able to achieve the kind of throughput which i
> should get from my RAID system. And i think the OS is the culprit
> (some bug in the vendor kernel) since for some reason it goes into
> huge iowaits preventing the applications to read/write more data. How
> can i debug whether the problem is with the OS or with the device?
>

#1 Problem usually: Is DMA enabled? (see hdparm refs)...

RAID FAQ & Docs...
http://www.faqs.org/contrib/linux-raid/x37.html
http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/sysadmin-guide/ch-raid-intro.html
http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch26_:_Linux_Software_RAID
http://www.tldp.org/HOWTO/Software-RAID-HOWTO.html

mdadm - tool for managing Linux Software RAID arrays
http://neil.brown.name/blog/mdadm

Benchmarking RAID configurations
http://www.catux.org/index.php?contingut=articles&num=101
http://www.acnc.com/benchmarks.html
http://www.coker.com.au/bonnie++/
http://www.iozone.org/
http://www.sourcepole.com/sources/reviews/raid/bottom.html

HOWTO Use hdparm to improve IDE device performance - GentooWiki
http://gentoo-wiki.com/HOWTO_Use_hdparm_to_improve_IDE_device_performance
HDparm - Tuning up your IDE hard disks using hdparm.
http://usalug.org/phpBB2/viewtopic.php?p=33142

-GL Art@JAX

--
Art Wildman - NWS JAX FL. - http://www.srh.noaa.gov/jax
"If you choose not to decide, you still have made a choice" -Rush|Freewill