SAS PCIe and expander math - is this correct

Can someone help me summarize some SAS math and assumptions

For this example an M1015 is a PCIe x8 SAS card and has two mini SAS connectors.

Each mini SAS can use a 8087 forward 4 way break out cable to connect directly 4 SATA or SAS drives for a total of 8 drives.

According to some product sheets this particular card provides:

Gives eight ( SATA 3.0 (6 Gbps) ports with forward breakout cable

Obviously that is non blocking as the port speeds vs spindle drive speed are high.

However because it is a SAS card each mini SAS port can be connected using a mini SAS to mini SAS cable to an expander card (for a total of two expander cards).

These expander cards come with further mini SAS ports allowing further chaining or many more drives using forward breakout cables.

So i we attach an expander card that has 6 mini SAS ports to one mini SAS port on a M1015 and populate the expander with forward breakout cables we have:

(6x4) 24 SATA drives ultimately connected to one mini SAS on the M1015.

So which of these is correct:

Maximum theoretical drive throughput is:

6Gbps /24 = 0.25Gbps per drive

or

(6Gbps x4) /24 = 1Gbps per drive

The second one.  If using the nicer chipset SAS expanders (eg, LSI based), they bond for lack of a better term the 4x 6Gb channel to the SAS HBA.  You don't have 4x 6Gbps channels you have one 24Gbps channel

JohnM posted somewhere on his Atlas thread that his drives were maxing out around 150MBps (1.2Gbps...), not an interface limitation but a hard drive speed limitation.  Using that math, a single 4x port off the M1015 can power between 20 and 21 drives full speed.

As an interesting side note, EMC has finally switched to SAS on their Clarion arrays.  The disk shelf loops are I am pretty sure 4x 6Gbps SAS.  SAS is high end stuff in comparison to SATA.

Thanks for taking the time to clear that up.

I am looking to upgrade my hardware to standardize on 2TB disks and increase parity speed as much as possible and this seems to be the missing X factor