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Date: Tue, 3 Feb 2004 17:03:02 -0800 (PST)
From: Doug White <dwhite@gumbysoft.com>
To: Cy Schubert <Cy.Schubert@komquats.com>
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cc: freebsd-current@freebsd.org
cc: Putinas Piliponis <putinas.piliponis@icnspot.net>
Subject: Re: atacontrol rebuild and iostat 
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On Tue, 3 Feb 2004, Cy Schubert wrote:

> > Why nothing ?
>
> Iostat doesn't see the I/Os because RAID rebuilds occur within the
> controller, the I/Os are not initiated in the O/S nor any of its utilities,
> therefore the FreeBSDS kernel doesn't see them. The O/S doesn't see the
> I/Os. Atacontrol see 3% because it specifically queries the controller for
> that information.

ATARAID is purely OS driven.  The OS issues the writes for the rebuild, as
well as failure detection and mirroring.  You're thinking of SCSI
controllers, or 3ware controllers.

Since the rebuild I/O is driven by the kernel, it bypasses the normal I/O
path and thus doesn't register in the stats.  If you try to do heavy I/O
to the devices, you'll find the performance is reduced.

> This is true of all hardware RAID controllers and true of all operating
> systems.

But not ATA RAID, which is Speshul. :)

-- 
Doug White                    |  FreeBSD: The Power to Serve
dwhite@gumbysoft.com          |  www.FreeBSD.org