Messages in this thread | | | From | Christophe Dupre <> | Subject | Re: Cheetah vs. UDMA: Bonnie says UDMA is faster! why? | Date | Wed, 25 Mar 1998 13:21:32 -0500 |
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> > 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 This is why I suspect that this section is out of date!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
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