Understanding Disk: Mechanical Limitations
October 28, 2013 6 Comments
Storage for DBAs: The year 2000. Remember that? The IT industry had just spent years making money off the back of the millenium bug (which turned out not to exist). The world had spent even more money than usual on fireworks, parties and alcohol. England failed to win an international football tournament again (some things never change). It was all quite a long time ago.
It was also an important year in the world of disk drive manufacturing, because in February 2000 Seagate released the Cheatah X15 – the world’s first disk drive spinning at 15,000 rotations per minute (RPM). At the time this was quite a milestone, because it came only four years after the first 10k RPM drive (also a Seagate Cheetah) had been released. The 50% jump from 10k to 15k made it look like disks were on a path of continual acceleration… and yet now, as I write this article in 2013, 15k remains the fastest disk drive available. What happened?
Disks haven’t completely stagnated. They continue to get both larger and smaller: larger in terms of capacity, smaller in terms of form factor. But they just aren’t getting any faster…
Published in 2004, this document shows that between 1987 and 2004 the performance of CPU increased by a factor of two million, while disk drive performance increased by a factor of just eleven. And that was over a decade ago, so CPU has improved many times more since then. Yet the 15k RPM drive remains the upper limit of engineering for rotating media – and this is unlikely to change, for reasons I will explain in a minute. But first, a more basic question: why do we care?
Seek Time and Rotational Latency
As we discussed previously, a hard drive contains one or more rotating aluminium disks (known as “platters“) which are coated in a ferromagnetic material used to magnetically store bits (i.e. zeros and ones). In the case of a 15k RPM drive it is the platter that rotates 15,000 times per minute, or 250 times per second. Data is read from and written to the platter by a read/write head mounted on top of an actuator arm which moves to seek out the desired track. Once the head is above this track, the platter must rotate to the start of the required sector – and only at this point does the I/O begin.
Both of those mechanical actions take time – and therefore introduce delays into the process of performing I/O. The time taken to move the head to the correct track is known as the seek time, while the time taken to rotate the platter is called the rotational latency. Since the platter does not stop spinning during normal operation, it is not possible to perform both actions concurrently – if the platter spins to the correct sector before the head is above the right track, it must continue to spin through another revolution.
Now obviously, both of these wait times are highly dependant on where the head and platter were prior to the I/O call. In the best case, the head does not need to move at all and the platter is already at the correct sector, requiring zero wait time. But in the worst case, the head has to travel from one edge to the other and then the platter needs to rotate 359 degrees to start the read or write. Each I/O is like a throw of the dice, which is why when we discuss latency we have to talk in averages.
Modern enterprise disk drives such as this Seagate Cheetah 15K.7 SAS drive have average seek times of around 3.5ms to 4ms – although in a later article I will talk about how this can be reduced. Rotational latency is a factor of the speed at which the platter rotates – hence the interest in making disks that spin at 15k RPM or faster. At 15,000 revolutions per minute, each revolution takes 4ms, meaning the set of all possible latencies ranges from 0ms to 4ms. Once you consider it like that, it’s pretty obvious that the average rotational latency of a 15k RPM drive is 2ms. You can use the same method to calculate that a 10k RPM drive has an average of 3ms and so on.
Why Not Spin Faster?
A surprisingly-common misconception about disk drives is that the head touches the platter, in the same way as a vinyl record player has a needle/stylus that touches the record (for any younger readers: this is how we used to live). But in a disk drive, the head “flies” above the platter at a distance of a few nanometers, using airflow to keep itself in place (similar to the concept of a fluid bearing).
There are three main problems with making disks that revolve faster than 15k, the first of which is energy: the power consumption required to create all of that additional kinetic energy, as well as the associated heat it creates. At a time when energy costs are constantly increasing, customers do not want to pay even more money for devices that require more power and more cooling to run.
A more fundamental problem is related to aerodynamics, in that by rotating the platter faster the platter’s outer edge starts to travel at very high speed causing it to interact negatively with the surrounding air (a phenomenon known scientifically as flutter). For a 3.5 inch platter, the outer edge is travelling at approximately 150 miles per hour. It isn’t possible to remove the air, because this would result in the head touching the platter – causing devastation. One potential solution is to reduce the diameter of the platter, e.g. 2.5 inches or less (thereby reducing the speed of the edge) but this results in reduced disk capacity. Some manufacturers are replacing the air with helium in an attempt to reduce flutter and buffeting, but helium is both expensive and scarce, while the manufacturing process is undoubtedly more complex.
But the biggest – and potentially insurmountable – problem is the cost. Technical challenges can (mostly) be overcome, but customers will not pay for products that do not make economic sense. Disks are built and sold in large volumes, so any new manufacturing process requires a heavy investment – essentially a gamble taken by the manufacturer on whether customers will buy the new product or not. It’s unlikely we’ll see anything faster than 15k RPM manufactured in volume, because there simply isn’t the demand. Not when there are so many cheaper workarounds…
As the old saying goes, necessity is the mother of invention. Give people enough time to work around a problem and you will get all sorts of interesting tricks, hacks and dodges. And in the case of disk performance, we’ve been suffering from the limitations of mechanical latency for decades. The next articles in this series will look at some of the more common strategies employed to avoid, or limit, the impact of mechanical latency to disk performance: over-provisioning, short-stroking, caching, tiering… But rest assured of one thing: they are all just attempts at hiding the problem. Ultimately, the only way to remove the pain of mechanical latency is to remove the mechanics. By moving to semiconductor-based storage, all of the delays introduced by moving heads and spinning platters simply disappear… in a flash.