SAS expander cascade to non-expander backplane?

So a bit of background, to help calibrate any responses, if anyone can get through the wall of text (I tend to be verbose). Skip to “Here’s where the question…” if you want to go straight to what I’m asking.

I handle IT and a lot of programming for my small company, which I’m a minority owner of. I’ve built a couple dozen servers over the past ~17 years. Presently, we have a nearly-full cabinet of colocated servers. One of the DB servers died (PSU is fine, MB won’t power up), so I decided 7 years of 24/7 operation is enough, and I’ve ordered parts for a new one.

I’ve been very partial to SuperMicro for a number of years, because I can usually find a good bare-bones server to start with (saving me build time), and I really like their rails (literally 2-seconds per corner to put in a standard square-hole rack).

I moved from Opteron to Xeon a while back, because the performance just wasn’t there anymore (first Opteron build was 2003). Now I’m moving back to AMD with EPYC. I’ve already put together three EPYC 1U servers, two with Naples (LVS load balancers), one with Rome (web server). I have a duplicate of the web server to build as well, and now the dead DB server to replace.

The old one is a 4U chassis with two Xeon E5-2630 v2 6-core processors and 32GB of RAM. Storage is 16TB on LSI MegaRAID (not interested in delving into something like ZFS right now).

I’m not asking for part advice, as I’ve already decided. I’m not going with bare-bones this time, because SuperMicro’s EPYC selection doesn’t have a good fit (nor does Tyan or Gigabyte). Here are the headline items from what I’ve ordered, for reference:

SuperMicro 829HE1C4-R1K62LPB 2U Chassis
SuperMicro H12SSL-i Motherboard
EPYC 7502p 32-core Processor
32GB 3200MT/s DDR4 Registered ECC RAM (x8 - 256GB Total)
SuperMicro AOC-S3108L-H8IR RAID Controller (rebranded LSI/Avago/Broadcom MegaRAID 3108)

Also getting 16 3.5" SAS drives and two 2.5" SAS SSD’s.

Here’s where the question comes in. Or, really, three questions, though they’re all related.

The chassis has two backplanes, or three if you count the 2.5" drive cage add-on going in the back. The primary one has an SAS expander chip, so a single 4-port SAS cable will provide access to all 12 drives connected to that. It actually has 16 ports on it, though, and supports cascading.

The secondary backplane is just four ports (have a look at the chassis to understand where it goes), while the third is just the 2.5" drive SAS/SATA cage in the back. So neither the second nor third backplane has an SAS expander chip.

I’m obviously going to find out the answers to my questions once I get the hardware, but time is important here (we don’t like operating with just a single DB server), so if I can get an answer before then, I can purchase additional needed hardware if necessary, sooner rather than later.

Question 1:

Can I connect the second backplane to the first one, and have the SAS expander chip on that first backplane handle the four drives connected to the secondary backplane?

Question 2:

Can I connect the third backplane as well to the first one, using another SFF-8643 port, and have the SAS expander handle all 18 drives?

Question 3:

Can I connect both remaining SFF-8643 ports on the first backplane to the RAID controller to increase bandwidth over just a single 4-port connection, without confusing either the backplane or the controller? The rear 2.5" drive cage will have SAS SSD’s in it, so it will make a performance difference, if it works.

The primary backplane is not dual-port. All four SFF-8643 connectors are both input and output for the same bus (for lack of a better term). There’s an alternate backplane available with eight such connectors, to allow redundant HBA’s, but that’s not what I’m doing.

So, can anyone tell me whether or not I made a bad choice, and will be scrambling to get another RAID controller?

Thanks in advance for any answers.

First, welcome to the forums.

The only way to really answer the question(s) is to know about the SAS-expander chip. First, find the part number on the chip, then find the data sheet for it. For untrained eyes, these datasheets are difficult to read, but given your history in IT I reckon you’d manage :wink:

Figuring out the expander chip just wasn’t going to be possible before getting the hardware.

As it turned out, things work perfectly with the configuration I wanted. The rear plane and top plane plug into the expander plane, and the expander plane plugs into the controller with two 4-port connectors. Everything is correctly recognized by the controller (including using 8 SAS lanes), and performance looks good.

Turns out the only real problem with the build was that the fan power cables were too short to reach the motherboard. Luckily I was able to get extensions for those very quickly from Amazon.

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