At some point, I had typed in a bit about the physical interfaces present on both the SAS and Fibre drives. I appears that I ran roughshod over that particular point which, upon thinking about it, is a very important dimension of drive reliability.
As noted previously, SAS drives use an amended SATA data+power connectivity schema. Instead of a notch between the data and power connections as present on SATA drives, SAS drives simply “bridge” that gap with an extra helping of plastic. This not only turns the somewhat flimsy SATA connectors into a more robust solution, it also requires that the host connector support that bridging. Interesting note here is that the SAS host connector supports SATA drives but SATA host connectors will not support SAS. This is somewhat assuaged by various host implementations (i.e. using a SAS connector on a backplane with discrete SATA data connectivity from the backplane to the mainboard) but generally, this is the rule. The SAS drives feature a male connectivity block which is mated to a female SAS connectivity block on the host system. Pretty basic stuff.
Fibre drives, on the other hand, use a SCA (single connector edge) medium that is again male on the drive side and female on the host side. Definitely more simplistic in design and implementation (and is featured within all current EMC arrays) and honestly, when push comes to shove, something I would trust inherently more with protection. The same idea is present with SCA80 Ultra320 SCSI drives as well. The fitment here is definitely more secure with less design stress placed on the physical connector (and thusly the PCB itself) than with the SAS solution.
There are always caveats with distinct designs, however, and I’d like to highlight some below.
a.) The SAS data+power connector is inherently MORE secure than the standard SATA interface. Truth be told, I’ve broken SATA data connectors. It’s really not hard since the data connection is a discrete “tab” from the power interface (which I’ve broken as well). The addition of the plastic “bridge” between data and power connections on SAS drives promotes a stronger bond between the connector (whether that be SFF or backplane based) and the drive itself. It also keeps folks from mistakenly connecting SAS drives to SATA ports. 😉
b.) The SAS interface is still prone to breakage as compared to SCA40/80 connections. There’s a reason why we do a conversion within our drive caddies from SATA to Fibre (outside of the obvious protocol translation and sniffer obligations): it’s more secure. The mating mechanism within the SCA interface provides no single point of stress on the connector as there is a nesting process that takes place. Not so with the SAS interface: you have a significant protrusion into the caddy area that, if improperly aligned, can cause damage. If you misalign the SCA interface, you can’t make the connection and there’s no protrusion difficulties.
Note: The good news in all of this (at least from my perspective @ EMC) is that we’re not going to allow you to screw this connectivity up. 😉 We mount the drives in our carriers, put them in the array and, well, we’ve got you covered. 😉
In any case, this is really for further clarification from yesterday’s post. Hopefully that will give a little more food for thought.
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