linux-kernel - Re: SATA RAID5 speed drop of 100 MB/s

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Date:	Sat, 23 Jun 2007 19:30:57 +0200
From:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
To:	Carlo Wood <carlo@...noe.com>
Cc:	Jeff Garzik <jeff@...zik.org>, Tejun Heo <htejun@...il.com>,
	Manoj Kasichainula <manoj@...com>,
	linux-kernel@...r.kernel.org,
	IDE/ATA development list <linux-ide@...r.kernel.org>
Subject: Re: SATA RAID5 speed drop of 100 MB/s


Hi,

On Saturday 23 June 2007, Carlo Wood wrote:

> PS I'd like to do extensive testing with Bonnie++ to tune everything
> there is to tune. But bonnie likes to write/read files TWICE the amount
> of RAM I have. It therefore takes a LOT of time to run one test. Do you
> happen to know how I can limit the amount of RAM that the linux kernel
> sees to, say 500 MB? That should be enough to run in Single User mode
> but allow me to run the tests MUCH faster. (I have dual channel, four
> DIMM's of 1 GB each -- 2 GB per Core 2 die. Hopefully the fact that
> I have dual channel isn't going to be a problem when limiting the ram
> that the kernel sees.)

"mem=" kernel parameter limits amount of memory seen by kernel
(more info in Documentation/kernel-parameters.txt)

You can also limit amount of RAM detected by bonnie++ by using -r parameter
but please remember that this will make bonnie++ benchmark combined kernel
I/O buffering + filesystem + hard disk performance instead of just filesystem
+ hard disk performance (as it can happen that some / all data won't ever
hit the disk).

Bart
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