linux-kernel - Re: limits on raid

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Message-ID: <20070618180327.GP10008@csclub.uwaterloo.ca>
Date:	Mon, 18 Jun 2007 14:03:27 -0400
From:	lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:	david@...g.hm
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, 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?

--
Len Sorensen
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