Date: Sun, 16 Jun 1996 16:24:21 +0200 From: Beck Peccoz Amedeo <gea@masternet.it> To: "Justin T. Gibbs" <gibbs@freefall.freebsd.org> Cc: freebsd-current@FreeBSD.ORG Subject: Re: ahc sincing questions Message-ID: <31C41915.41C67EA6@masternet.it> References: <199606160635.XAA19559@freefall.freebsd.org>
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Justin T. Gibbs wrote: > > While starting up the kernel with the -v option the ahc driver > >reports the sinc speed for any attached scsi device in MHz. > >Shouldn't be in Mbytes/s? > > MHz is correct. The sync rate is a measure of the clock rate at > which data is transfered. Only when you multiply that by the bus > width, do you get MBytes/s. Ok, I see. > > Why I see no difference when enable the ultra speed? My HD can > >sinc at 20 Mbytes/s, but it actually sincs only at 10... what's > >wrong? > > If you have a Wide drive, it does 10Mhz sync on a 16bit bus. If you > have an 8bit device, it must do 20Mhz sync in order to do 20MB/s. > What is your drive model? The only Ultra drive I know of on the market > is a Seagate one, and the person who tried it said it refused to negotiate > 20Mhz sync (looking from a SCSI bus analyzer) and that it was the device's > fault and not the driver's. I've got a Seagate wide HD (ST32550W) acceptinc max sync transfer rate of 20 Mbytes/s (i.e. 10MHz). If I set the ultra speed on the adapter (20MHz or 40Mb/s) it still syncs at 10MHz. So far so good. A problem arises when I run a benchmark, a very silly one, that does only read a big unfragmented file, as I get a transfer rate of only 7.4 Mb/s, while I read of a test on the same drive/adapter leading to a transfer rate of 16Mb/s, more than twice. It seems that the HD only negotiates at 10Mb/s, and as the ahc reports 10MHz either one of theese value is wrong or the wide channel is only used as a normal 8 bit one. -- Beck-Peccoz Amedeo GEA Software S.r.l. Via Deffeyes, 1 11025 Gressoney Saint Jean (AO) ITALY Tel. ++39-125-366302 Fax. ++39-125-366415
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