[ILUG] ITX advice

[paul at clubi.ie]

I understand why SCSI is used a lot for RAID, and I agree with you 
that reliability is only one small reason.

You need to stop conflating IDE and SATA though (e.g. SAS is SATA phy 
with SCSI command protocol over it - you sometimes can plug SATA 
drives into SAS bays).

On Tue, 11 Sep 2007, Michael Watterson wrote:

> The reason for SCSI is:

> Decent HW RAID (most SATA/IDE "RAID controllers" are really SW RAID)

This hasn't been true for many years.

> Scaling: Easy to have lots of drives

This isn't true, even if measured against IDE. You can have lots of 
IDE ports quite easily.

> Cheap Clusters: Easy to share SCSI drives between two controllers if 
> different Host IDs

a) Electrically shared buses have certain problems.
b) It doesn't scale

Shared storage tends to be done with either SANs or network storage..

> Performance for multiuser (RAID 5, striping on a single bus works, 
> IDE/SATA you need a separate controller for each drive to have 7 
> drives executing different commands at once.

See second paragraph. This is waffle for SAS v SATA, where either you 
use a SATA phy / disk, or you have a SATA multiplexer.

For parallel-SCSI, the shared bus is a SPOF. (Same for SATA/SAS 
extenders, but you have a choice there).

> while a replacement drive is obtained.  I doubt SATA is yet 
> supporting true HW RAID5 and and Hot spares.. I could be wrong.

a) RAID has nothing to do with SATA/SCSI
b) You'd be wrong

regards,
-- 
Paul Jakma	paul at clubi.ie	paul at jakma.org	Key ID: 64A2FF6A
Fortune:
He's just a politician trying to save both his faces...