The SAS (r)evolution …

January 16, 2014

When someone within the company started discussing the evolution of SAS a couple of weeks ago, I thought I misheard them … was that “revolution” or “evolution”? Turns out it’s been a bit of both. In early days it was a revolution, but now that it is a well-matured technology we are within the “evolution” section of its lifecycle.

At the core of all of this technology is the venerable SCSI command set … which turned out to be just about the most long-standing and solid technology developed within the storage industry. Developed over 30 years ago, it is still running strongly today across many and varied delivery mechanisms.

So since we are talking about SAS, and not the SCSI command set (which is used in a lot of places in storage today), let’s look back, at today, and at tomorrow to see where we’ve been, are and are going.

When serial first came along, my first thoughts were … “thank goodness for simpler cabling”. Didn’t really turn out that way as yes, we didn’t need termination like on the old parallel bus, but we ended up with a truckload of new cables, and probably even more than we had in the old scsi days. However, 1.5Gb down the pipe sounded pretty good, and indeed early performance was great right out of the box.

Adaptec made a decision back in those days to build both SAS and SATA controllers, but we quickly worked out that we could do it all with one controller (SAS) because SATA is a subject of the SAS protocol, and we only need to make one controller to talk to all disk drives at the same time. While SAS jumped straight into the performance end of town, early SATA implementations were pretty dodgy and somewhat slow – and really held things back until SATA II and SATA III came along.

So 1.5Gb … wow, that’s pretty fast. In fact it’s faster than most spinning disks can go even today. Hmmm, don’t think we’ll need an upgrade for a while. No, as usual we had to have an upgrade, and so we got 3Gb. Well that had to be it, surely … spinning media will never catch up with this (and indeed it hasn’t today), but wait, what are those funny little things called SSD? OK, let go to 6Gb … the old “double it and they will come” principle. Great, now we’re cooking. Overkill for spinning drives but what the heck, we’re keeping up with the SSDs … or so we thought. OK, so your SSD can do more than 600Mb/sec … then well go to 12Gb and gloat about our performance (for a while).

And so here we are – 12Gb SAS. Interestingly we haven’t had the complications or growing pains of the old parallel SCSI – shorter cables, different connectors (oh yes, that’s right, we did do different connectors for 12Gb), but generally the evolution has seen less pain and therefore quicker uptake than the previous parallel regime. That said 12Gb is very new and we don’t really know the uptake of it yet. In fact the 12Gb standard has brought some very nice negotiation processes with it for device handling so in that respect it’s a step forward in both functionality and speed over 6Gb SAS.

But is it enough?

For the moment it will have to be … and for a couple of reasons. The PCIexpress bus is capable of somewhere in the vicinity a theoretical 8000Mb/sec, while our 24-port 6Gb SAS chips can hit a theoretical 9600Mb/sec. Note the theory side of things because the fastest I’ve ever achieved out of our controllers is 6,600Mb/sec (wow!). So there is no real advance to PCIe3 on the horizon so we’re not going to get any degree faster there.

There are also some new kids on the block – SCSI Express and NVM Express which drive the storage device across a PCIe bus rather than a SAS bus – and this “may” be the way forward instead of an ever-increasing level of SAS (school is still out somewhat on exactly what the future holds in the interface market just yet).

The bit I really find ironic is that no-one I’ve heard of is really asking for more Mb/sec than they can get today with a bunch of 6Gb SSD. It’s all about IOPs and latency. If we are talking 4Kb blocks, then it takes an awful lot of those to saturate a bus capable of handling 6000Mb/sec … it will generally be the processor that flood and bottlenecks before the bus in this scenario. Latency is proving a problem child for the SAS world which is one of the claims to fame of the SCSI Express/NVMexpress world … since they are PCIe both sides of the controller they claim to have lower latency – and that is very, very appealing to datacenters today – it’s all about latency and IOPs.

So, on this “evolutionary path” of SAS, have we made it to the end? I might have been thinking so until I received a phone call this morning from a bloke looking for a SCSI card … and I was reminded that SCSI/SAS etc in all its formats and variations over the years as outlived more than one computer tech J

Don’t know about you but I’m looking forward to learning new interface, new technology and new way of putting together yet another complex solution for a customer!




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