What I learned today: Daisy-chaining Hardware Port Multipliers, June 26, 2012

If you're like me, you grew up in a world where, for the most part, if you wanted to know something, you googled it. Discounting those early elementary school projects where they forced you to learn how to use an encyclopedia, the vast majority of the information that I ever looked for I found on the internet, for better or for worse. The modern world would be slow and impractical without the web's hive mind - in fact, you count on other people to post somewhere that they, too, have encountered Windows Error Message 7x000f941A42 (but only when booting from an external drive, on a Sunday, with a Japanese version of iTunes installed and playing a cover of "Brown Eyed Girl"). So when I went looking for a particular piece of information on the internet today, and didn't find it (!), it threw me for a loop.

So if anyone out there is wondering if you can daisy-chain SATA hardware port-multipliers (HDMs) and see the disks connected to the second HDM as individual drives (JBOD*), the answer is...no. You can't. Sorry.

Do you not know if you have this problem? Here's a handy little diagram:

All stand-alone letters here represent drives.

What we have here is a computer with a particular multi-drive compatible SATA PCI Express card installed. Into that, we plug Hardware Port Multiplier number one. Into that, we plug (some number of hard disks and) a second Hardware Port Multiplier. We connect some number of hard drives to this second HPM. Questions: can you see the disks at the second level? do they appear as individual drives or one big drive?

Answers: If this is the setup you have, and you're trying to configure ABCD as JBOD (i.e., you want them each to be their own volume), you're SOL - it violates the protocol of how the information is transferred. ABCD can (must), however, be configured as RAID (you would see them as one logical volume). See this helpful tutorial from Addonics for more on that. abcd can be configured as RAID or JBOD - I think. You should verify that with someone who's more of an expert than me. Have you checked the internet?




*RAID = Redundant Array of Inexpensive Disks
JBOD = Just a Bunch of Disks
Learned in the context of TechnoFrolics, a very cool company that does very cool stuff.