Subject: Speed of Ultra SCSI compared to Ultra DMA?
To: Jonathan Belson <>
From: David Gilbert <>
List: tech-userlevel
Date: 10/02/1998 18:57:57
>>>>> "Jonathan" == Jonathan Belson <> writes:

Jonathan> Hiya A kind soul recently gave me an Adaptec 2940 Ultra SCSI
Jonathan> card.  I was wondering how much difference in performance I
Jonathan> would get if I moved to SCSI from my current UDMA setup.
Jonathan> SCSI drives seem to have higher rpms on the whole, also
Jonathan> there's the issue of CPU usage during transfers.
Well... to compare, the 6.4 gig UDMA 7200 Quantum medalists are giving
me about 4.5 meg/s on my FreeBSD system.  On an identically configured
FreeBSD system, except for the 2940 (narrow ultra) and 6.4 gig Quantum
7200 SCSI drive, I get up to 14 meg/s.

These measurements are both through the filesystem with large blocks.
The SCSI actually seems to be quite variable.  Writes go 7-8 meg/s and
reads go 12-14 meg/s.

Where you win, however, with SCSI is the 'tagged command queueing' ---
where the controller can send more than one command to more than one
drive.  Your 2940 (last I checked) supports 16 outstanding commands.
Drives vary... some support 1 or 2... some 4 or 8.

Where this helps is when the drive is doing many small transfers
(which is the majority of HD traffic IMHO).  It means that the drive
(or drives) can be always busy.


|David Gilbert, Velocet Communications.       | Two things can only be     |
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