OK, we’ve talked about SAS drives and the fact that what people really want out of them is “fast”. Let’s expound a bit.

What anyone wants out of an HDD is access density; IOPS/GB, and a High GB/$ (more normally specified as low $/GB, but I inverted this to make a point on the following graph).

Unfortunately we can see from this that as GB/$ has gone up sharply, access density has dropped precipitously. What is access density? Access density translates to the number of actuators chasing data. Thus with the relentless pursuit of more GB per disk drive, access density has gone in the tank.

We all know that the low capacity models of server drives last a lot longer than a mere $/GB consideration can explain, and the reason is that the more capacity you have under an actuator, the lower the performance of a subsystem comprised of these sorts of disks.

So 24 spindle “shelves”, or in Pillar’s parlance - Bricks, have higher access density than the same capacity, low RPM larger platter incarnations. Hence, better performance.

There are a few interesting and very enlightening points you can make about this:

1. Access density is always at odds with cost for HDD-based subsystems of a given RPM.

2. Access density always gets better with smaller platter sizes for an equal number of platters.

3. Smaller form factor drives of the high RPM variety don’t yield as big an improvement as you might think, because 95mm (3.5” form factor) 15K RPM drives already use smaller platters – closer to the 2.5” form factor that the SFF HDD uses anyway

4. What Small Form Factor Drives give you is the ability to package them in a serviceable enclosure that puts more actuators per TB in the familiar storage tray! Serviceability is key for small, medium, and large Enterprise.

5. There has always been an option of stacking drives in some high density fashion to optimize actuators per unit volume, but Serviceability is just about always compromised – hence it is not a common practice.

OK good, now the question is… what else can we do to get around access density? Well, with legacy architectures the answer is cache (don’t use the disk if we can avoid it). We can pay more and use smaller disks – oh, the wonderful days of 9GB disk drives.

Or, we could take a large capacity disk with great $/GB, and short-stroke the disk. As an example, let’s say we use 10% of the drive’s capacity. The access density will go up by about 20X!! (~2X from access time reduction, 10X from the fact that the actuator is chasing 10% of the capacity). Wow!. And to think we made that HUGE improvement by only increasing the $/GB by 10X. Oops.

Well, here is what the Axiom QoS does for you. It allows you to short stroke the drive, and get a HUGE access density improvement. But instead of throwing away the other 90% of the capacity, it allows you to sneak in and access that part of the disk in a way that only causes the access density of the high performance capacity (10% share) to drop by say, 10%, from 10X better to only 9X better. WOW! You get the whole drive space back, but have a portion of it that behaves like it has 9X the access density!!

You see, the truth is that stove-piped storage doesn’t get you anything if you can buy a single array that can mitigate contention while increasing access density.