Jack -
The recommendation refers to the backend disk loops that connect the shelves to the controllers. You *can* put a SAS shelf and a MSATA/NL-SAS shelf in the same loop from the controller. This will affect performance as a whole (SAS would be slowed, MSATA/NL-SAS doesn't get faster) with an appropriate workload as the slower disks responses *could* limit the ability to drive and work with the faster disk responses.
Best practice is for each disk type (not necessarily shelf type, but it usually boils down to shelf type as well) to be in a separate disk loop to the controllers. For instance, SSD should be in a different loop than high performance SAS even though they might both be on DS2246 shelves, again because of the different IO performance nature of the disks.
In practice, I will mix on a single loop disks of similar IO characteristics but different capacities - for instance 600GB/1.2TB 10000RPM disks might be on the same loop. I tend not to do this by default, but will if time/space/port require. This type of concern comes into play usually as total controller ports are near being exhausted and I don't have freedom to power down and add port cards. It also highlights why pre-planning on controller deployment is important, as is the ability to predict the future 2-3 years out. Anyone who mastered that one please let me know.
Hope this helps
Bob Greenwald
Lead Storage Engineer | Huron Legal
Huron Consulting Group
NDCA | NCIE-SAN Clustered Data OnTap
