what is the difference between a sas controller and a sas raid controller?

Question

What is the difference between a sas controller and a sas raid controller? Are they the same thing, just named differently, or is the SAS Controller purely a connection point with no RAID capabilities and the SAS Raid Controller is something that can be configured (somehow?)

Solution 1

A SAS controller (HBA, Host Bus Adapter) on its own has no RAID ability. It just presents however many disks are plugged in to the underlying operating system, and it's the OS's job to figure out what to do with them.

A RAID controller has a processor (and it should also have some RAM) embedded on the card itself. That RAM acts as a cache, and the processor does RAID functionality. The number of disks that the underlying OS sees depends on how you configure your RAID volumes.

A SAS HBA typically also supports non-disk based devices, such as a tape drive. RAID controllers typically do not.

A SAS or RAID controller is typically backwards compatible with SATA disks, but whether or not you can mix SATA and SAS on the same card depends on the card.

If you are using a modern software storage solution like ZFS or Windows Storage Spaces then a SAS HBA (non-raid) may be all you need, at a much lower cost.

Note that you may be limited by the speed of the PCIe lanes or the RAID controller in both these cases. A mid-tier RAID card usually caps out at around 700MB/sec, whereas just a SAS controller can probably saturate an 8x PCIe link (~7000MB/sec if the disks on the other end can keep up).

Solution 2

A SAS HBA presents just a bunch of disks to the server independently. This is, indeed, where the acronym JBOD comes from: just a bunch of disks. If RAID is desired, the OS must provide it with software RAID.

A SAS RAID controller doesn't present the disks independently (unless configured to do so); rather it provides hardware RAID service, and presents RAID arrays as virtual disks to the server.

Comments

[shodanshok]

Possible duplicate of serverfault.com/questions/616831/…

[Michael Cropper]

That sounds straight forward enough - thanks for the clarification.