The Future of Disk Storage


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Taller hard drives with multiple actuator arms supplied in multi-drive packages — these are just a few of Google's suggestions as it calls for a complete rethink of storage disk design.

In a white paper called "Disks for Data Centers," published last month, the company gives some hints as to how the hard disk drive might evolve in the coming years.

There's a need for change, the white paper asserts, because the fastest-growing use case for hard drives is for mass storage services housed in cloud data centers. YouTube alone requires 1 million GB of new hard drive capacity every day, and very soon cloud storage services will account for the majority of hard drive storage capacity in use, it says.

Not Cloud-Optimized

That's significant because hard drives are not — and never have been — designed with these cloud storage services in mind. Far from it: enterprise grade disks have been optimized for use in servers, while standard consumer grade disks are designed for use in business and home PCs.

Here's why Google believes that's a problem. In servers and desktop machines it's important that hard disks don't lose data, so one of the design goals is that they have a low bit error rate (BER). Achieving this goal has an associated cost, which is reflected in the cost of the disk.

But when data is stored in cloud storage services, it is always stored on multiple disks as a matter of course to ensure availability. That means minimizing the BER is unnecessary. In fact, it’s an undesirable goal because when BER is minimized cloud providers are paying for reliability that they don't need because the issue is taken care of elsewhere.

"Since data of value is never just on one disk, the bit error rate for a single disk could actually be orders of magnitude higher than the current target, " the report says.

Avoiding paying for disk drive features (such as reliability) that are not needed is important to cloud storage service providers because of the vast numbers of disks that they buy and use. Other features that are important to cloud storage service providers include high performance (in terms of IOPs) and high capacity.

They also have different security requirements, which stem from the fact that in cloud storage many different people's data may be stored on the same physical disk. Most enterprise drives offer data-at-rest encryption using a single key, but in a shared service there's a need for more fine-grained control using different keys to access different areas of the disk.

Disk Collections

A key to designing better disks in the future is recognizing that data center disks are always part of a large collection of disks. That's important because any future designs need to optimize various metrics — specifically IOPs, capacity, security requirements, TCO and tail latency, Google believes — but these metrics need to be optimized for the collection of disks, not the individual disks that make up the collection.

That means that target levels of capacity and IOPs can be achieved by using a specific mix of drives, and new disks added to the collection as capacity requirements increase can be tailored to bring the collection closer to these targets.

New Shapes

So the obvious question is this: How will data center drives, optimized as collections, differ from today's enterprise hard drives?

One promising avenue to explore is changing the form factor, Google believes.

One way of lowering capacity costs ($/GB) would be to increase the diameter of the platters inside a disk drive from 3.5" to perhaps a 5" disk. But the problem with that solution is that it decreases the performance (IOPs/GB) as there is a greater area for a head to move over.

Shrinking the platter size would increase $/GB but increase IOPs/GB, because there is less area for the head to move over, and smaller platters are also more stable and can therefore be spun faster to provide better performance.

The remaining option would be to change the height of a new disk drive. A standard 3.5" hard drive is up to 1" tall, but this figure was derived from the form factor of PC floppy disks and could easily be changed.

"We propose increasing the allowable height (of a drive)," Google's report says. "Taller drives allow for more platters per disk which adds capacity and amortizes the costs of packaging, the printed circuit board and the drive motor /actuator."

The report adds that it may be that a mix of different platter sizes, in different disks, provides the best aggregate solution.

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