Is there currently a mechanism for constructing a hardware RAID array using storage devices that have a full-size PCIe slot, PCIe M.2, or PCIe U.2 bus interface, such as SSDs?

It is extremely unclear to me how this might work, since a normal hardware RAID has the drives directly plugged into the RAID controller via SATA or SAS interfaces, but meanwhile these new kinds of storage devices plug straight into the system PCIe bus without a separate drive/device controller.

I can foresee two possible hardware implementations:

  • Pure PCIe x16 or x8 RAID controller with no onboard drive interface connections - communicates with the separate storage devices via the standard system PCIe main bus. Data transfer speeds may be limited due to a lack of available PCIe channels between the RAID controller and each individual PCIe storage device. Also the RAID controller lacks exclusive access to the member drives, which seems like it could be a data integrity/security problem.
  • RAID controller with it's own secondary PCIe bus on the card, with a special PCIe bus interface cable extending out to an external PCIe slot cage that may resemble a traditional SCSI/SAS hot plug backplane - the PCIe storage device cards plug into this isolated PCIe bus, can ONLY communicate with the RAID controller, and have no direct path to the system CPU or memory.

(As of writing this, searching Server Fault for "U.2 RAID" or "M.2 RAID" turns up nothing, and I'm creating the tags "U.2" and "M.2". Is no one already doing this?)


I am now trying to accomplish what you are proposing on an aging Dell. I also wondered how it's possible, although I read somewhere that newer mainboards are designed to boot from PCIe pairs configured for RAID. I have not confirmed this.

My solution, though not perfect, was to mount my M.2 drives in enclosures that have the 2.5" SSD form factor and U.2 ports, and then connect them to a high performance RAID card via SAS. I haven't delved into the transfer rates, but I'm presuming that they are far superior to regular SSDs configured for RAID and connected via SATA.

I'm not out of the woods yet, however. I'm now looking for cables to connect all of this. But at least you might consider this avenue if you haven't already found another solution that works.

  • It is unclear to me if U.2 is compatible with SAS, whether electrically, or by protocol. So much info about it is very vague. – Dale Mahalko Oct 2 '19 at 8:02

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