From owner-freebsd-fs  Thu Sep 21  8:23:37 2000
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From: Soren Schmidt <sos@freebsd.dk>
Message-Id: <200009211526.RAA98589@freebsd.dk>
Subject: Re: disable write caching with softupdates?
In-Reply-To: <8133266FE373D11190CD00805FA768BF055BD1D7@shrcmsg1.tdh.qntm.com> from Stephen Byan at "Sep 21, 2000 08:15:50 am"
To: Stephen.Byan@quantum.com (Stephen Byan)
Date: Thu, 21 Sep 2000 17:26:17 +0200 (CEST)
Cc: gibbs@plutotech.com ('Justin Gibbs'), mbendiks@eunet.no,
	fs@FreeBSD.ORG, sos@FreeBSD.ORG, freeBSD-scsi@FreeBSD.ORG
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It seems Stephen Byan wrote:
> Justin Gibbs [mailto:gibbs@plutotech.com] wrote:
> 
> > > Without write caching, you pay one disk rotation for each 
> > sequential write.
> > 
> > This should not be the case if you are allowed to overlap 
> > commands.  The
> > only penalty should be increased latency in seeing a write complete.
> 
> You're correct. I was writing with respect to ATA drives, of which I believe
> only IBM's support write queuing, so I overlooked the case where queuing is
> available. 
> 
> I'm not that familiar with ATA in practice; the spec for ATA queuing looked
> sufficiently convoluted (i.e. a kludge) that it wasn't obvious to me that it
> would be a performance win to implement it.

Well, but it is, I see 5-10% performance gain using tagged queing, so its
definitively worth the trouble. I dont think it is that bad, it was easy
enough to implement support for in the ATA driver, but so fa I think we
are alone in actually using it :)

-Søren


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