I'm still kinda new to SCSI, but I have a question regarding the
bandwidth of SCSI vs SATA.


U320 SCSI has a bandwidth per channel of 320 MB/s correct?  Where you
can have up to 16 drives, so the bandwidth would be split between
channels.


SATA has a bandwidth of 150 MB/s per channel, meaning if you have a 4
port SATA board, each drive (assuming there are 4) would get the full
150 MB/s?


So lets say you have 4 drives, 4 SATA, 4 U320 Scsi.. It would seem to
me that SATA would outperform the SCSI ones.


If you had 2 drives, it would seem that SCSI would outperform SATA?

Basically, it is my understanding that SCSI is shared bandwidth, much
like IDE, distributing U320 over x # of drives on a chain, whereas SATA
has dedicated 150 on each drive.

Am I correct on these statements?


Thanks!

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> SATA has a bandwidth of 150 MB/s per channel, meaning if you have a 4
> port SATA board, each drive (assuming there are 4) would get the full
> 150 MB/s?

The data rate "under the head" of the drive (at least SATA drive) is much
lesser then 150MB/s. For instance, my 160GB SATA Seagate drive (usual commod
ity
one) has a data rate of 56MB/s on outer tracks and 28MB/s on inner tracks.

So, the 150MB/s limit is just not relevant.

When you compare SATA drives to SCSI ones, you must not only consider the
interface itself, but also the drive internals. IDE/SATA and SCSI drives dif
fer
not only in interface. They are absolutely different.

The situation is that the enterprise-class drives are usually implemented wi
th
SCSI interface, while the SOHO drives - with IDE or SATA. There are rare
exceptions like WD Raptor, but they are rare.

For instance, SOHO drives are not designed for 24/7 _head use_ - though they
are designed for 24/7 spin-up. Enterprise-class drives are surely designed f
or
24/7 head use. And so on.

Note that even the usual capacity figures for SCSI disks are not the same as
for IDE/SATA products.

> So lets say you have 4 drives, 4 SATA, 4 U320 Scsi.. It would seem to
> me that SATA would outperform the SCSI ones.

The "under the head" data rate of the drives is important here. Rotation spe
ed
is also important (7200 on SATA, 10K or 15K on SCSI). Seek time too, and so 
on.

I think that even sharing the U320 SCSI cable across 4 disks can give the
higher data rates then 4 independent SATA controllers.

> Basically, it is my understanding that SCSI is shared bandwidth, much
> like IDE, distributing U320 over x # of drives on a chain, whereas SATA
> has dedicated 150 on each drive.

Correct.

--
Maxim Shatskih, Windows DDK MVP
StorageCraft Corporation
maxim@storagecraft.com
http://www.storagecraft.com

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