Re: [TCLUG:12134] SCSI vs. IDE

[Thomas Veldhouse <veldy@visi.com>]

Many of the modern SCSI drives are the exact same piece of hardware as the
SCSI equivalent.  The only difference is a SCSI interface chip on the
drive.  So why the extreme difference in cost?  

Scsi drives support multithreaded access (via the different chipset) while
IDE does not.

I am not sure about "offloading" the work from the CPU to the drive - as
it is usually handled by sending commands and filling buffers -
non-blocking.  

There is a great comparison on Tom's Hardware site if you care to look.

Tom Veldhouse
veldy@visi.com

On Wed, 12 Jan 2000 Nick.T.Reinking@supervalu.com wrote:

> >Now these ata66 controllers, how do they make the landscape different?  It
> >seems like they have scsi-like drivers in windows.. dont know about linux
> >support.
> 
> Well, ATA/66 is basically ATA/33 but with a maximum transfer rate of 66MB/sec, 
> and higher data integrity.
> This "data integrity" depends on having a special (i.e. expensive!) 
> 80-conductor IDE cable.   Instead of having
> a normal 40-pin IDE cable, you have to use an 80-pin IDE cable.  It's basically 
> the normal IDE cable except
> for every other wire on the cable being a ground, and having one wire punctured 
> at a special (28th wire?)
> spot.  It's very unusual - and the cables are *very* expensive - even more so 
> that UW internal SCSI cables.
> Now you IDE  folks now what it's like for us who use SCSI.  ;)
> 
> > Your comment about CPU indicates that SCSI is better for even
> >one drive.  This mirrors an experience I had when I was running a heavy
> >test that read and wrote random 8 meg segments of a 64 meg file.  My
> >machine slowed down to a crawl and all the CPU time was being spent on
> >'system'.
> 
> Well, IDE can be very CPU efficient - *if* DMA is enabled for your drives.  (It 
> has to be compiled into the
> kernel).  If your IDE controller isn't using DMA, you're limited to PIO 4 
> transfer rates - which can be limited
> either by the maximum transfer rate (modern IDE drives can hit that limit 
> pretty easily), or by the amount
> of CPU power you have - pushing about 17MB/sec can really tax your CPU, so it 
> might have been a CPU
> barrier that you ran into.
> 
> Be sure to compile in DMA - or get SCSI!
> 
> Nick Reinking
> 
>