From owner-freebsd-mobile  Fri May 11  8: 9: 6 2001
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To: Leon Breedt <ljb@devco.net>
Cc: freebsd-mobile@FreeBSD.ORG
Subject: Re: slow IDE performance 
In-reply-to: Your message of "Fri, 11 May 2001 11:28:08 +0200."
             <20010511112808.E8710@shell.devco.net> 
Date: Fri, 11 May 2001 08:08:43 -0700
From: "Kevin Oberman" <oberman@es.net>
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> Date: Fri, 11 May 2001 11:28:08 +0200
> From: Leon Breedt <ljb@devco.net>
> Sender: owner-freebsd-mobile@FreeBSD.ORG
> 
> I'm not sure if this was already posted, a search didn't turn anything up.
> 
> On my Thinkpad A20m I experienced insane IDE write slowdowns at some point
> in updating to 4.3-RC. I could basically do *nothing* while doing large IDE
> transfers. This persisted in -RELEASE and -STABLE.
> 
> A quick query of my friendly committer colleague revealed that ATA write
> caching was now disabled by default.
> 
> Enabling it in /boot/loader.conf restored my sanity:
> 
> hw.ata.wc="1"

Yes, there was quite a long discussion of it about a month or two
ago. Look in the archives of stable for the subject "Significant disk
(ATA) slowdown with recent stable". 

The reason that disk cache was disabled is that many IDE disks are junk
and don't deal with cache properly in many respects. As a result, use
of write cache is placing the integrity of disk data in doubt and can
lead to disastrous failure in the event of unexpected power failure.

I see a 400% increase in the time required for some disk write
operations with write cache disabled, so I bit the bullet and enabled
it on my laptop. It does have a good battery backup, after all, but be
sure that you understand the risks involved in the use of write cache
on cheap disks.

It is VERY dangerous to use write cache on a desktop or server where
there is no battery backup.

R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman@es.net			Phone: +1 510 486-8634

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