SCSI--LVD vs SE...

[Chris Dillon]

On Sun, 7 Dec 2003 lists at battleface.com wrote:

> Am I understanding this right?...
>
> Concerning SCSI, are LVD and SE (single ended) mutually exclusive?

Yes, unless a SCSI bridge is used to separate the LVD and SE devices
on the same SCSI bus.  If any SE device is present on an LVD bus, the
entire bus will revert to SE.

> That is, if I have a system that supports LVD drives, I want to
> configure those drives for LVD and not SE which would yield a
> smaller bandwidth?  Like 1/2 at best, right?

Being configured as LVD should happen automatically as long as
everything you connect to the bus is LVD.  You can achieve 40MB/sec at
best with a Wide SE bus.  A Wide LVD bus allows speeds of 80MB/sec up
to 320MB/sec depending on the attached controller and devices.

> Finally, I have a Tyan S2468UNG motherboard with Ultra160 SCSI
> onboard (dual channel). I'm looking at pairing two Seagate 15K.3
> (ST336753LW)  drives with this board. Are these a good choice? Would
> I get better performance from having both drives on the same channel
> (and so, cable)? or would it be better to put one on channel A and
> the other on channel B (separate cables)?

I doubt you would see much difference putting just two drives on two
different Ultra160 channels, but if you have the channels and extra
SCSI cables to spare, go ahead and use them.

> In the future I would like to add a third LVD drive (probably the
> same model with larger capacity). Again, all on the same channel? or
> split to second channel? I'm guessing the dual channel approach
> would yield better performance when writing across separate physical
> disks.  Especially if three drives are involved.

Again, on an Ultra160 channel, three new and very fast drives will
come close but still will not fully use the available bandwidth.
Spreading the drives across two channels can help with contention
under very heavy loads, but depending on what you are using this for,
you may see no difference at all using two channels rather than one.
SCSI is far better at doing multiple devices per channel than IDE.

-- 
 Chris Dillon - cdillon(at)wolves.k12.mo.us
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