+1 for me on this - thanks for the post!
I have had my ASUS board with PIKE slot in service with a Supermicro card I thought was LSI but is Marvell. Waited around and now the PIKE 2008 adapter card is $30 on ebay (from China) so thought I'd update the post a bit as I make the move to PIKE, and add a few more links that came up while I was researching making it EFI friendly.
Pic and Description page of the PIKE 2008 Basically it's the LSI 9220-8i (or IBM M1015), all LSI SAS-2008 chip based.
LSI was acquired 2014 by Broadcom. So most searches will point you back to Broadcom for drivers (tho should be functional out of box with virtually any OS). But some firmware updating might be pertinent.
LSI Search - start with "LEGACY" as your Product Group ---> Legacy Raid Controllers in the search
I was also able to find a live (non-end of life) Broadcom SAS 9210-8i product that's LSI SAS-2008 based, has driver and Firmware link that's active, which might be smart thought for newer Linux Kernels. Who can guess after all what Intel will be messing around with microcode wise trying to react to the recent vulnerabilities. I/O streams are probably not sacred from being spared I'm guessing. 9210's a Host Bus Adapter (vs. RAID centric card), but that'd be more appropriate for UnRaid anyway if it flashes - it's using the same I/O 2008 chip for sure so I may try it when my card arrives, I'm guessing it will work.
https://www.broadcom.com/products/storage/host-bus-adapters/sas-9210-8i#tab-archive-drivers3-abc
One helpful hint might be to consider putting your cache SSD's on other MB SATA ports since for some older MB's these are only PCIe 2.0 x4 or 2.0 x8 ports, so could bottleneck with several SSD's going at once. Obviously not as relevant if it's a PCI 3.0 x 8 port.
Probably going to look something like this in UnRaid
03:00.0 Serial Attached SCSI controller: LSI Logic / Symbios Logic SAS2008 PCI-Express Fusion-MPT SAS-2 [Falcon] (rev 03)
Pics for installation for the curious...
A non-battery backup low height RAID card that can do JBOD, nicely misses the pricer SAS server aftermarket crowd AND frees up a slot on ASUS boards that have the PIKE slot, so kind of a UnRaid perfect storm. Definitely more convenient SATA port placing (why don't all MB's do it like that?)
For any doubters, ASUS confirms the PIKE is an LSI SAS 2008 (ASUS refers you directly to LSI website). It's SAS II (meaning cross compatible for SATA III drives @ 6Gbps). Not UEFI friendly out of the box from what I'm reading, but also seemed that the flash updates mentioned above may improve upon that if that's important to you.
ASUS PIKE FAQ page
Fast Facts about the LSI SAS 2008
The RAID controller supports both SAS 2 and SATA III at 6.0gbps
Approximately 9w of power consumption for common cards
Single PowerPC core at 533MHz
No onboard cache
PCIe 2.0 x8 interface
Supports SAS expanders (with dual linking)
Uses sas2flash utility to flash to IT/IR mode (when possible)
Here's a page showing other analagous OEM's based on the LSI 2008 (can likely save money using one of those if PIKEs not in your future after reading this). the 2008 LSI cards Seem to be one of the most popular historical RAID cards out there.
servethehome.com info for OEMs
Flash advisements from Broadcom
I have the ASUS dual xeon Z9PA-D8 board, just boosted up the CPUs to E5-2667 V2's, for much better single thread / VM performance from the 2.6Ghz E5-2670s, which kind of lags a bit in VMs. Sho I should be futureproofed for awhile now, meaning these boards and V2 Xeons might be an ebay target for value conscious folks - mine is an ATX sized board which made it nice to get into a smaller and very quiet case. The board has USB 3.0, meager onboard VGA (Aspeed which I think is an ARM chip?), has onboard sound and a weird tiny little iKVM board support add in card. PIKE SATA/SAS slots are conveniently at the edge of the motherboard and vertically connect for cleaner cabling, well out of the way of the GPUs (why don't all boards do that?)
***FYI, for those with a PIKE slot collecting dust that don't need another SATA/SAS card, the front port can also function (allegedly) as an extra PCIe slot it's a PCIe 4x slot turned around backwards), so if you happen to have a card that could do something for you in that config with some jury rigging, etc it might be worth a look. For example maybe a SATA expansion port card that you already have lying around, or a card with USB headers, or maybe USB 3.1 card comes to mind if bracket removed, it would be amenable to some gymnastics with a port adapter/extension cable, etc. I'm in NO WAY guaranteeing this to work for you or advocating it, just noting that the rearward of the 2 slots actually is PCIe spec. with opposite orientation, and have read posts where folks have used PCIe devices in it successfully.