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Date: Thu, 3 Sep 2009 10:21:06 +1000
From: Emil Mikulic <emikulic@gmail.com>
To: Alexander Motin <mav@FreeBSD.org>
Message-ID: <20090903002106.GB17538@dmr.ath.cx>
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Cc: "Derek \(freebsd lists\)" <482254ac@razorfever.net>,
	FreeBSD-Current <freebsd-current@freebsd.org>
Subject: Re: siis/atacam/ata/gmirror 8.0-BETA3 disk performance
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On Wed, Sep 02, 2009 at 05:51:35PM +0300, Alexander Motin wrote:
> To completely load gmirror on read operations, you may need to run
> two dd's same time. Also make sure, that your gmirror runs in
> round-robin mode. Default split mode, which should help with linear
> read, is IMHO ineffective, at least with default MAXPHYS and slice
> values.

On that note, there is an excellent patch in this PR which improves
the way gmirror schedules read requests to different disks:

http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/113885

Could someone please commit this?

With this patch and a two-way mirror, I can run two linear scans of
different files in parallel and get almost perfect scaling.  (result:
this approximately halves the wall-clock time it takes to do a backup of
some fat VM images)

IIRC, without the patch it's faster to run them sequentially.  :(

--Emil