Re: Cheetah vs. UDMA: Bonnie says UDMA is faster! why?

Christophe Dupre (cdupre@risq.qc.ca)
Wed, 25 Mar 1998 13:21:32 -0500

>
> Thanks to all who responded. The result: I accepted the default in
> the synchronous transfer frequency and should have bumped it up.
>
> Would it be unsafe to make the driver default to 20 MHz instead? That
> would avoid this problem from other first-time NCR + UW users.

I have no experience with the NCR hardware or driver, but have administered
many boxes with SCSI. Here's my experience.

> I just went and re-read the help in this section and it had a few
> things to say that are misleading:
>
> > Specify 0 if you want to only use asynchronous data transfers.
> > Otherwise, specify a value between 5 and 10. Commercial O/Ses
> > generally use 5 Mhz frequency for synchronous transfers. It is a
> > reasonable default value.
>
> This is why I went with the default. Are asynchronous transfers a
> bad idea? I am not familiar with this aspect of SCSI.

asynchronous transfers are a bad idea performance-wise. It's slow, but can be
used over really crappy SCSI cabling. If you're down to using async SCSI, just
go buy new SCSi cables.

5MHz is the max speed of the SCSI-1 specification. All devices should be able
to use that speed (except maybe very old tape backup that will negociate
slower rates).

>
> > However, a flawless singled-ended scsi bus supports 10 MHz data
> > transfers.

10 MHz is Fast-SCSI and SCSI-2 maximum speed. At this speed, you need good
cables but can live without proper termination.

20 MHz is ultra-SCSI. It needs premium cables & termination.

> Hmm. This is why I suspect that this section is out of date! The new
> NCR 895 even supports 40 Mhz... I wonder how many drives will support
> this. 8^)

40 MHz is Ultra2-SCSI. It's the latest in term of SCSI, and there's not yet a
lot of support for it, at least affordable for the basic sysadmin. Also, if
not used with differential SCSI, the maximum cable length is quite small - 4
feet or so.

As far as the hardware is concerned, you should not have any problem using 20 MHz.

-- 


Christophe Dupre
Analyste de systemes, 
RISQ inc. ;-)
1801 McGill College, suite 800           Tel: (514) 840-1235, ext 6971
Montreal, QC CANADA                      FAX: (514) 840-1244


"Nous ne sommes pas libres de ne pas etre libres, nous sommes obliges de 
l'etre"  -  Fernando Savater


#include <disclaimer.h>






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