Date: Thu, 30 Aug 2001 00:19:33 -0500 From: Mike Meyer <mwm@mired.org> To: Trevin Chow <tmchow@sfu.ca> Cc: Mike Meyer <mwm@mired.org>, <questions@freebsd.org> Subject: Re: Performance tuning and adding RAM Message-ID: <15245.52453.614096.245669@guru.mired.org> In-Reply-To: <20010829215734.F79672-100000@benny.geektank.org> References: <15245.36122.131328.798913@guru.mired.org> <20010829215734.F79672-100000@benny.geektank.org>
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Trevin Chow <tmchow@sfu.ca> types:
> On Wed, 29 Aug 2001, Mike Meyer wrote:
> > The reliability of soft updates is dependent on knowing that data
> > written to disk is actually on the disk. If you turn on softupdates,
> > turn off write cache on your disk drives.
> I'm confused by your last statement. Why do I turn off write cache on my
> HDs? Are you talking about another sysctl variable or in teh actual bios?
I'm talking about a (mis)feature that some hard disks have. To quote
the tuning(7) man page:
With IDE write caching turned on, IDE hard drives will not only
write data to disk out of order, they will sometimes delay some of
the blocks indefinitely when under heavy disk loads.
For IDE drives, you disable this by setting the sysctl hw.ata.wc to 0,
but it has to be done from the boot loader at boot time. If you are
running 4.3-RELEASE, it's set off by default. If you're running
something prior to 4.3-RELEASE, you may not be able to set it at
all. The drive manufacturer may provide a DOS utility that will let
you turn it off as well.
If you are using SCSI disks, you can use the camcontrol modepage
command to check and disable it. It's the WCE value on page 8; use
"camcontrol modepage daX -m 8 -e" to start an editor, change the 1 to
a 0, and then exit.
<mike
--
Mike Meyer <mwm@mired.org> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
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