Fixed Storage Transfer Rates (was: RE: SX & LX Board Compatibility)

[Brett Andrews <taukimus uci edu>]

Ok.  I am simply trying to help here...  I don't want anyone else watching 
to get confused.
These are the transfer rates for ATA-type controllers:

ATA:
PIO Mode 0			3.3 MBytes per second
PIO Mode 1			5.2 MB per second
PIO Mode 2			8.3 MB per second
PIO Mode 3			11.1 MB per second
PIO Mode 4			16.6 MB per second

DMA:
DMA-2				16.6 MB per second
UltraDMA/33:			33 MBytes per second

And these are the transfer rates for SCSI Controllers:
SCSI-1 (Narrow)		5 MB per second
SCSI-2				10 MB per second
Ultra SCSI			20 MB per second
Ultra Wide SCSI		40 MB per second
Ultra2 Wide SCSI		80 MB per second

All of these transfer rates are defined (in various places: Adaptec, for 
instance) as BUFFER TO HOST BURST TRANSFER rates.  These are not sustained 
transfer rates.  I believe that the real problem for the SX/LX IDE onboard 
controllers is that they do not support UDMA, nor are there really good 
drivers for Linux for these controllers.  Because the drivers for the 
onboard controllers run in PIO mode, they take up a lot of CPU time (this 
is because of the nature of PIO mode transfers).  Bus master UDMA (33 
MBytes/s) controllers can achieve speeds (and low CPU utilization) nearing 
UltraWide SCSI (40 MBytes/s) for a lot less money (controller and drive) Of 
course, drivers that support UDMA would be necessary.

I think the actual sustained transfer rate you get from a hard disk depends 
on how the drive was manufactured and the RPM's and access times....

Anyway, this is for anyone else who was getting confused by the way people 
were mixing Mbits with Mbytes, and burst with sustained transfer, when 
quoting transfer rates.

Hope I Helped,

Brett Andrews
taukimus@uci.edu

On Friday, May 08, 1998 1:45 PM, Chris Price [SMTP:cprice@missing.its.to] 
wrote:
> I think that some of the comments that have been made about ide vs scsi
> (various flavors) is influenced by the fact that the LX/SX IDE drivers
> are shite, whereas NCR (and to a lesser extent)and Adaptec drivers are
> extremely mature and 'feature rich'.
>
> 	I for one believe (based on 2 years running a high load INND server)
> that the NCR based cards have better linux performance, especially since
> the merging of the BSD/Linux driver development tree for the NCR
> chipset.
>
> > I don't agree with your assessment of "narrow" SCSI.  Ultra-narrow
> > controllers can still achieve 20MB/s, better than the best EIDE 
available
> > on Intel (which tops at out 16MB/s IIRC).  This is more than adequate 
for
> > desktop systems.
>
> 	UltraDMA 33 is 33mbits/sec (burst of course). SCSI (narrow or wide ) is
> a much better choice for those seeking or needing sustained transfer
> rates above 20-25mbits.sec.