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Date: Wed, 25 Oct 2006 15:24:55 +0200
From: Guido van Rooij <guido@gvr.org>
To: "Patrick M. Hausen" <hausen@punkt.de>
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Cc: Rink Springer <rink@freebsd.org>, freebsd-stable@freebsd.org
Subject: Gmirror performanc (was Re: Gmirror question)
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On Wed, Oct 25, 2006 at 12:48:29PM +0200, Patrick M. Hausen wrote:
> You don't need to.
> 
> If your bsdlabel partition is N sectors in size, the gmirror
> object will have size N - 1. Newfs will not be able to write
> to that last sector. You newfs the finished mirror device,
> not the individual partitions.

Indeed. How stupid from me.

Anyway, I created a gm device and a partition. Now the read performance
is not what I'd expect.
I have the partition on two SATA devices on different controlers.
I get around 60MB/s for each disk. I can get that speed from both disks
simultaneously.
Now when I dd from the gm device, I don't get any speed higher than that.
I tried with -b split -s <various sizes>, -b round-robin, -b load.
(dd-ing as done with a bs of 1m; I see the transaction size is 128Kb,
unless the split method is used, in which case the transaction size
gies down. When round-robin is used, the transaction size is 128Kb/s,
but the number of transaction per second goes down.).

I cannot explain why I should not get a higher read speed. Anyone?

-Guido