I'm learning quickly, but in order to advance I need to get this part straight.
A single physical mini-SAS port provides 6 Gbps of bandwidth (let's forget about the slower 3 Gbps please). That amounts to 750 megabytes per second. Factoring in the 8/10 bit encoding this becomes effectively 600 megabytes per second. Connecting 1 hard drive to this mini-SAS port is obviously not all that efficient.
I also see no problem in grouping 4 drives to a single mini-SAS-port. However, things change when an expander card comes into play. Let's assume the expander card can connect 20 drives (. The expander card is connected to the Raid controller using a single SAS 6 Gbps link. I smell a bottleneck here.
So, can I "bundle" 2 (or more!) physcial SAS-links so the expander card has a 12 Gbps links to the raid card (minus encoding)?
From your SAS Card (RAID or just SAS-HBA), a 4x Mini SAS connector is ~600MBs x4 to the SAS Expander. x4 is the normal amount of lanes from a host to an expander. Say you have 24 drives off of the expander. Each one is 100 MB/s (more if there is a large cache on drive). Then 2400MB/s of the drives combined = bandwidth of your x4 SAS Pipe. Remember, that SAS is full duplex, so the drive spin up and reading of the drive will still be the bottleneck.