Backplane to HBA connection confusion

[Ahrimanes]

Im planning to build a TrueNAS server of my old HPE DL380 GEN8 25-SFF. I have 24 SAS12 SSDs and plan to pair these with a LSI 9305-24i HBA.

From my understanding this backplane supports 25 SAS12 disks, but here is where I get confused. There are only two SFF-8087 ports on the backplane. How is it possible to connect these to a HBA and present 25 disks to it? I have found SFF-8087 to SFF-8643 cables, but Im not sure this will work. From my understanding a single SFF-8087 can carry 4 disks?

Backplane pn: 686568-001
Cable pn: 686614-001

Hopefully there is someone here who can explain this to me.

Edit: spelling

 

[mattventura]

The backplane has a built-in SAS expander. It's like a network switch but for SAS.

 

[Ahrimanes]

The backplane has a built-in SAS expander. It's like a network switch but for SAS.

Thanks, I suspected something like this. But that would also mean I will loose bandwidth, right?
Another question, is a xxxx-24i overkill since I can only use 2 ports? Would a xxxx-8i do the job as well?

 

[kapone]

But that would also mean I will loose bandwidth, right

Each of those SAS ports is 6gbps x4. If you connect both ports to your HBA = 48gbps total bandwidth (not quite, but it's more than 24gbps and somewhat less than 48gbps. This is called a SAS wide port configuration)

So, unless you're using SSDs and plan to run ALL of them at the same time, yes, you'll lose bandwidth.

is a xxxx-24i overkill

Yes.

Would a xxxx-8i do the job as well

Yes.

 

[Ahrimanes]

@kapone @mattventura Thank you for taking your time to answer my questions. I have now ordered two SFF-8087 to SFF-8643 cables. I might not be able to get full performance from my 24 SSDs in striped mirror, but probably it will be more then enough for my needs.

 

[bonox]

are you sure they'll work. SAS is not NVMe and I'venever heard the expression SAS12 NVMe. NVMe is pci express. SAS is a protocol of its own and I don't think tri-mode NVMe capable HBAs appeared in the 9300 series lineup

 

[Ahrimanes]

are you sure they'll work. SAS is not NVMe and I'venever heard the expression SAS12 NVMe. NVMe is pci express. SAS is a protocol of its own and I don't think tri-mode NVMe capable HBAs appeared in the 9300 series lineup

You are absolutely correct, its me thinking one thing and typing another. My disks are SAS12 SSDs nothing else. I will edit my post.