Question to the Storage Advisors, from FrederickO: I heard that the external SAS cables will have direction orientation. Can you explain why and will there be markings to let you know which way to go?

In my last post I referenced a SAS webinar intended to cover basic SAS topics. As is common with these things, the attendees are able to ask questions during the webinar and we try to give live answer for as many as possible. When we run out of time, as we did this time, we try to follow up with answers.

Luckily, I didn’t get to one the questions, namely the one at the top of this post.

If I had answered it live, I would have embarrassed myself by basically responding “WTF?!”. I would have gone on to explain how SAS transceivers have the same characteristics regardless of whether they’re in the initiator, expander or drive. Luckily the person asking the question couldn’t have responded to my answer, else the millions (or maybe it was hundreds) of listeners would have heard me called an idiot.

After the call one of my apparently much smarter coworkers, MikeL, reviewed my answers to the left-over questions. He pointed out that, yes, the new Mini-SAS connectors are keyed and the cables are directional. Huh?!

That sent me on a rollicking romp through the SAS spec, which I am now going to share with you, based on the hope that I’m not the only one that didn’t know about Mini-SAS connector keying. If you already knew about this keying, for my sake please just play along and pretend that you’re hearing it for the first time.

The root of the issue is that expander ports have three types of addressing: direct, table and subtractive.

Direct is used to directly connect to a drive or initiator. The expander simply has to remember that address “a” is attached to port “1”, for example.

Table is used when that port connects to another expander that has multiple devices behind it. Therefore port 1 has access to addresses a, b, c, etc.

Subtractive is used when all else fails. In other words, if the expander didn’t know where address x was, then it would send it to the subtractive port. Obviously there can only be one subtractive port per expander.

The following diagram shows a configuration using one initiator daisy-chained to two JBOD expansion boxes. The blue box is a JBOD containing an expander represented by a yellow box. Each expander uses all three types of addressing.

As you can see, the first expander knows that drives a, b, and c are directly connected to three of the ports. This is known as direct addressing.

The expander also knows that drives d, e, and f are in a different enclosure via the “out” port. The mapping on this port is done via table addressing.

Lastly, the connection back to the initiator is via the remaining port using subtractive addressing.

Now hopefully it’s obvious that the user configuring this system needs to know which ports are in (subtractive), and which are out (table). This is done by keying the cable connectors and receptacles. The SAS spec defines three types of keys: In, Out and End Device. The following cable receptacle drawings show the three keying methods.

End Device Receptacle

Out Receptacle

In Receptacle

Likewise, the cable connectors also have one or more keys.

End Device or Out Cable Connector

End Device or In Cable Connector

Additionally, each cable connector has an icon to indicate the type of key.

IN – circle
OUT – diamond
END DEVICE – both circle and diamond

So, that’s the story of Mini-SAS cables and connectors, and why they actually are directional. Fascinating. Tomorrow we’ll explore wire gages, and the age old mystery of why larger diameter wires have smaller numbers. Someone - shoot me now. Please.

TT

This entry was posted on Wednesday, November 30th, 2005 and is filed under Storage Interconnects & RAID, Advisor - Tom Treadway. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.