This is just a quick primer on the primary internal SAS cables you’ll find when buying used servers of the Xeon E5 series and later. I’m going to skip the less common cable connectors like SFF-8482 which is more often seen on SAS Controllers used in personal compute environments. The connectors covered here are also referred to as “Mini-SAS”.
Quick Concepts
Just to avoid making this too lengthy, here’s a few quick concepts to give you a better understanding of how SAS works, without going into a deep dive.
- SAS controllers offer multiple SAS channels down a single cable, usually 4 channels.
- These channels are then negotiated and split across the connected devices
- Thus SAS can be expanded via a SAS expander (one cable in, multiple cables out), and multiple cables just add to the number of available channels to be used.
- A backplane on a server is basically just a SAS expander with disk compatible headers
- This is in contrast to SATA which is a one-for-one relationship
- Bigger SAS controllers give more channels
- There are also external SAS options to connect to disk only shelves, but they’re not covered here. They’re basically giant SAS expanders.
- So a backplane that has two connectors and 8 drive slots will often work on any random four slots with a single cable plugged in. More cables = more channels that aren’t directly relational.
SFF-8087
This is the most common SAS connector found in servers. It has now been replaced by SFF-8643 in the same way that USB-C has replaced USB-A. The new one “replaces” the old one in air-quotes. However for consistency and compatibility, most large server manufacturers are still using SFF-8087.
The most important thing to note is that both types are completely pin-compatible. So if you buy a non-OEM RAID card, such as the LSI SAS3008 which has the newer SFF-8643 connectors, you can still connect these to an SFF-8087 backplane.
Wide-SAS (36 pin/Double Wide)
This is a frustrating beast of a connector. Technically it’s an SFF-8087 cable that’s doubled up and carries twice as many channels, as seen on the right on the image above, but I’ve only ever seen it on Dell and HP machines and subsequently if you’re looking for cables of a certain length, you have to sift through part numbers from those two brands. You’ll usually find this on Dell/HP PCI-E RAID cards of a couple generations, so if you buy, say, an HP P440, it will normally have this connector on it.
Technically it will probably slightly improve routing or airflow in a small chassis like 1U, but if I were to guess, I’d suggest it’s just a money grab from Dell/HP.
SFF-8643
This is the current miniSAS connector, intended to replace SFF-8087 and it’s significantly easier to use than SFF-8087. However it’s not commonly found on servers. You’ll encounter this connector when using modern SAS cards (such as the LSI SAS3008) and is a pin to pin match to the SFF-8087 allowing you to get replacement cables for your server with SFF-8643 on one end, and SFF-8087 on the other.
So if you find yourself putting a modern SAS card in an older server, this is the connector you’ll probably encounter and you’ll just need to pick up replacement cables to fit it.
Cable Length
Often overlooked when looking for add-on or replacement cards for servers. Servers, as you probably are aware, are all shapes and sizes and so are the cables to match.
- Too short a cable and you’ll spend hours Googling and maybe weeks waiting for the right size cable to arrive.
- Too long a cable isn’t a huge drama, but it can obstruct airflow or generally be hard to fit
- Or worst of all, a dual head cable from a wide-SAS header where one fits great and the other can’t reach the second header on the backplane.
Generally it’s worth just taking a quick measurement of your server based on how you would route the cable, and get one that is at least that length. Often storage controller cards will come with cables, and often these won’t be the right length, so by all means buy a card with cables, but assume you’ll need to buy more cables and/or will get a nice surprise when it fits.
Summary
There’s two common SAS headers, and one extra used in current server environments. You should get value out of simply knowing their names and of their existence, but expect that regardless of whether a card includes cables or not, you’ll probably need to buy more to suit your specific environment and they can be mixed and matched.
Finally, use as many cables as you can fit, but you can get started even if you can’t fill them all due to how SAS channels are used across devices.
0 Comments