SAS Expander

[Gandalf Corvotempesta]

Sorry for the stupid question but I have a supermicro SSG-6027R-E1R12T server with integrated RAID controller and 12 disks.
I would like to not use the integrated raid and I've added a AOC-S3008L-L8e SAS HBA.

Vendor told me that with a single, four-lane cable, connected to the integrated SAS expander and a single HBA port, i'll be able to use all of 12 disks.
I've tried and I'm able to see all of 12 ports, but the question is: how this is possible ? The HBA only has 8 internal ports (where I can connect 2 cables) but supports 122 devices.

is a single cable able to see 122 devices by using a single backplane? Vendor told me yes, but I would like to know how this is possible.

Vendor also suggested me to connect both HBA ports to both backplane ports, doubling the total bandwidth in case i would use SSDs.

Can someone confirm if this is true and explain me why?

 

[Ericloewe]

how this is possible ?

The backplane has an expander. Think of it as an Ethernet switch, but for SAS.

Vendor also suggested me to connect both HBA ports to both backplane ports, doubling the total bandwidth in case i would use SSDs.

This is a bit optimistic, but theoretically true.

Switching four lanes to 20 or eight to 16 makes no real difference to the SAS expander, theoretically. So you should be able to just connect two cables to double the bandwidth to the expander.
In practice, this may not always work, unfortunately. But it should.

 

[Gandalf Corvotempesta]

So a single cable is able to connect 12 disks?

Why on specs is wrote that each HBA port is able to connect 4 disks?

How can I ensure that all is working properly with 2 cables? Any tests to run?
Is as simple as: if you see all slots with 2 cables then everything is working?

I don't have 12 disks right now so I don't want to have some bad surprises in the future. If I start with 2 cables but just using the first 3 slots, when I have to use the remaining 9 slots I don't want to power down or anything similiar

Any bandwidth test to run and see if two cables are better than one?

Last question: with two cables, any SAS disk (that is dual port) is automatically connected to both SAS port on the same controller? In this case, a cable failure or badly plugged, doesn't bring the whole array down

And what if I put two HBA cards by connecting each port to a single backplane port? I'll get hba redundancy automatically?

I have many pci slots available and this HBA is relatively cheap, I can put 2 cards on each server, if useful.

 

[Ericloewe]

Why on specs is wrote that each HBA port is able to connect 4 disks?

That's without expanders.

How can I ensure that all is working properly with 2 cables? Any tests to run?
Is as simple as: if you see all slots with 2 cables then everything is working?

Use a heavy workload.

Last question: with two cables, any SAS disk (that is dual port) is automatically connected to both SAS port on the same controller? In this case, a cable failure or badly plugged, doesn't bring the whole array down

No, that's a completely different scenario.

And what if I put two HBA cards by connecting each port to a single backplane port? I'll get hba redundancy automatically?

No, you need dual expanders.