linux-kernel - Re: limits on raid

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Date:	Mon, 18 Jun 2007 11:12:45 -0700 (PDT)
From:	david@...g.hm
To:	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>
cc:	Brendan Conoboy <blc@...hat.com>, Neil Brown <neilb@...e.de>,
	Wakko Warner <wakko@...mx.eu.org>,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org
Subject: Re: limits on raid

On Mon, 18 Jun 2007, Lennart Sorensen wrote:

> On Mon, Jun 18, 2007 at 10:28:38AM -0700, david@...g.hm wrote:
>> I plan to test the different configurations.
>>
>> however, if I was saturating the bus with the reconstruct how can I fire
>> off a dd if=/dev/zero of=/mnt/test and get ~45M/sec whild only slowing the
>> reconstruct to ~4M/sec?
>>
>> I'm putting 10x as much data through the bus at that point, it would seem
>> to proove that it's not the bus that's saturated.
>
> dd 45MB/s from the raid sounds reasonable.
>
> If you have 45 drives, doing a resync of raid5 or radi6 should probably
> involve reading all the disks, and writing new parity data to one drive.
> So if you are writing 5MB/s, then you are reading 44*5MB/s from the
> other drives, which is 220MB/s.  If your resync drops to 4MB/s when
> doing dd, then you have 44*4MB/s which is 176MB/s or 44MB/s less read
> capacity, which surprisingly seems to match the dd speed you are
> getting.  Seems like you are indeed very much saturating a bus
> somewhere.  The numbers certainly agree with that theory.
>
> What kind of setup is the drives connected to?

simple ultra-wide SCSI to a single controller.

I didn't realize that the rate reported by /proc/mdstat was the write 
speed that was takeing place, I thought it was the total data rate (reads 
+ writes). the next time this message gets changed it would be a good 
thing to clarify this.

David Lang
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