How to use SAS Expander with RAID card?

Question

I've doing some researches since few days and I haven't really got an answer to my question.

I am in the process of building a huge file server for the company I work for. We bought a server case NORCO RPC-4224 4U rackmount server case with 24 bays and drives. Now I am stuck at the point where I have to choose the RAID card. I saw a RAID card from LSI that offer 6 internals mini-SAS 8087 ports (6x4) and would work perfectly with my build. However, it's not cheap.

I have been reading about the subject and I found something called SAS Expander. They are cheap. HP is selling an expander card with 6 mini SAS 8087 for 225$ (instead of 1150$ of the other one).

Now my question, how would I link an SAS Expander with a RAID card ? Do I need to a buy a RAID card with only 1 external 8088 port and link it with the other card ? Could someone clarify this for me because I haven't found any good documentation on it.

Thanks you.

Solution 1

There's a lot of good information here about the HP SAS Expander.

Question that may help you are:

Do SAS expanders work transparently with SAS controllers?

How exactly does a SAS SFF-8087 breakout cable work? + RAID/connection questions

But in general, yes, you can use the HP SAS expander to link one controller to more disks than 2 SFF-8087 ports would natively provide. This, of course, only applies to internal SAS disks.

For the enclosure you linked, there are 6 SFF-8087 ports and no oversubscription. A SAS expander will be needed.

The definitive resource for the HP expander card you're interested in is here.

Solution 2

Basically yes, you do either take a card with external ports or use a slot bracket like this one to connect an existing controller with internal-only ports:

The HP expander card would need to be powered through a PCIe slot. You could use an empty mainboard (no need for CPUs or RAM) attached to the PSU(s) of the chassis to make it work.

This article over at servethehome.com describes a project which is very similar to what you are trying to do. But to be honest, what you should be doing is buying a JBOD with the expander (preferably even two of them) already built into the backplane. You might want to look at Supermicro's SC846E26 for an example.

You also should do thorough testing with the controller / expander combination - expanders do not work (all that well) with SATA drives, so you might want to plan for SAS interposers as well. Additionally, specific RAID controllers may or may not work with expanders - make sure to check the spec sheet and bug the presales support.

Comments

[David Bélanger]

Thank you for your reply. I'll look into what you gave me and see what I can find.

[ewwhite]

This is all for one enclosure, right? Everything is internal.

[David Bélanger]

Yes, we have one enclosure. I was thinking of putting the RAID controller with 1 external 8088 and the SAS Expander with 6 8087 Internal + 1 External 8088 and link both with 8088 cable using both external. I haven't found any hardware that would do this from the inside... I know about nothing about SAS and still reading on it.

[ewwhite]

Then you need the HP expander card.

[David Bélanger]

Let say I get the RES2SV240 Expander and I get a RAID controller with 2 8087 port. All I have to do is connect 1 backplane to my RAID controller, connect all my others 5 backplanes to my Expander and the port left on the Expander will connect on my RAID card. In the end, my RAID card will see all 24 drivers and I will be able to use RAID on them assuming there's no compatiblity issues ?

[David Bélanger]

Ok so if I understand correctly, the expander doesn't need to be connected to the RAID card, only the PCI port ?

[ewwhite]

EVERYTHING goes into the Expander.

[David Bélanger]

Ok so like I said, even the RAID card goes into it. I made a super picture of it: imagebam.com/image/f05f9a273907161

[the-wabbit]

@DavidBélanger Why is "SAS1" connected to the controller? For the sake of consistency, it should be connected to the expander as well. The RES2SV240 seems to support it too - specs explicitly state "Eight SFF8087 SAS/SATA connectors".

[ewwhite]

@syneticon-dj Exactly... I tried to stress that everything is connected through the expander.

[David Bélanger]

Ok, I understand that, but will the RAID card see all 6 SAS even if they are not directly connected ? Everything is linked not by the SAS cable but by the motherboard's BUS ? So from what I understand, I connect all 6 SAS into the Expander and nothing in the RAID card ? From my PoV it doesn't mae sense because the RES2SV240 draw only power from the PCI lane. The RES2SV240 act like a switch and the RAID card has a router from what I think and understand.