Date: Thu, 12 Nov 1998 18:22:38 +1030 From: Greg Lehey <grog@lemis.com> To: "Justin T. Gibbs" <gibbs@narnia.plutotech.com> Cc: hackers@FreeBSD.ORG Subject: Re: SCSI vs. DMA33.. Message-ID: <19981112182238.J463@freebie.lemis.com> In-Reply-To: <199811111538.IAA00103@narnia.plutotech.com>; from Justin T. Gibbs on Wed, Nov 11, 1998 at 08:38:43AM -0700 References: <98Nov11.134648jst.21907@ns.isi.co.jp> <199811111538.IAA00103@narnia.plutotech.com>
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On Wednesday, 11 November 1998 at 8:38:43 -0700, Justin T. Gibbs wrote: > In article <19981111194213.H20849@freebie.lemis.com> you wrote: >>> I must say that the newer UDMA IDE drives has come a long way >>> lately, they perform at least as good as their SCSI counterparts. >>> The only thing that they cannot do is overlapping commands, but >>> given EIDE's much smaller cmd overhead, I'm not sure this has >>> any significance at all in practice. >> >> This last point would appear to be borne out in my measurements >> earlier today. > > You had all of 1 command going to each disk. That doesn't give > you any per-device overlap. Sure. I was referring to the command overhead, not the effect of overlapped commands. > If you really want to see the effect of overlapped commands, run a > benchmark through the filesystem that causes lots of commands to be > generated. Do it with tagged queuing and without. If your devices > support a reasonable number of transactions, the effect of disabling > tagged queuing on latency is quite dramatic. I once introduced a > bug into CAM that effectively disabled tagged queuing. For several > days I couldn't understand why my interactive performance was so > lousy during large compile runs. I think that the testaments on > this list and others about the dramatic improvement CAM has made to > the performance of high load, random seek, workloads also shows the > effectiveness of overlapped I/O. The main reason CAM performs so > well is the order of magnitude increase in the number of concurrent, > per-device, transactions the system supports. No doubt, and that's what I intended to do next. Unfortunately, I've just fallen off the net (massive phone cable damage out in the street), so I don't can't download any benchmarks. Greg -- See complete headers for address, home page and phone numbers finger grog@lemis.com for PGP public key To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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