How to Buy a Hard Drive: An Essential Guide

Posted 08/24/2011 at 1:54pm | by Loyd Case

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Do you need gigabytes or performance? Laptop upgrades or a screaming new gaming PC? We walk you through what you need to know to pick the right storage solution for your PC.

Storage. Always needed, often overlooked.

Often lost in the buzz surrounding the latest DirectX 11 GPUs and hexacore CPUs is the ability to actually store and retrieve your stuff. Your applications, games, photographs, digital music and everything else lives on your hard drive. But that boring old rotating magnetic disk just doesn’t seem exciting or high tech – even though the technology in a hard drive is actually pretty incredible.

One technology that has made storage a little sexier is SSDs – solid state drives based on flash memory technology. But SSDs aren’t a perfect solution, as we’ll see shortly. We’ll cover the gamut of storage options for your OS and apps to help you better understand what storage solution is best for your needs. (Note that we’re not going to talk about optical storage, which really is secondary these days.)

We’ll first touch briefly on technology and jargon, then look at several different scenarios, and try to focus on what storage options might be appropriate and cost effective. But first, let’s talk tech. We’ll first briefly discuss hard drives, then take a quick look at SSDs.

Good Old Fashioned Hard Drives

Hard drives consist of small platters coated with magnetically sensitive material. These platters are designed to be stacked up to five high, and run at spin rates up to 15,000RPM. Some high end desktop hard drives max out at 10,000RPM, but most performance hard drives for desktop PCs run at 7,200RPM. The 15K RPM drives are mostly used in servers.

10,000RPM. That’s really fast rotational speed. It’s about as fast as you can get for a desktop hard drive.

One of the key aspects of hard drives is areal density – how many bits you can cram onto a square inch. Despite the relative maturity of the technology, hard drive makers continue to improve on areal density. Seagate and Samsung have both announced hard drives that will ship in late 2011 that offer one terabyte per platter, or 625 gigabits per square inch.

Magnetic heads, mounted on arms that are moved with an electric motor called an actuator is how the data gets written to and read from the disc. Head technology is as critical as areal density, because the heads have to be extremely sensitive to read individual bits when there are 625 billion of them in a square inch of disk space.

There are several key aspects to hard drive performance:

• Areal density. The more bits you can cram onto a platter, the faster the drive, all other things equal. At the same spin rate, higher bit densities means that the more data is read off the drive per linear inch as it spins.
• Spin rate. As you spin a drive faster, more bits travel under the head, and more data read each second.
• Cache. Most hard drives have some DRAM cache. More cache is generally better. The highest performance hard drives have as much as 64MB of fast DRAM cache.
• Head technology. The robustness and responsiveness of the motors that move the head (the head actuator) determines how quickly the head can be moved to different areas of the drive. This affects random access performance.

One thing that doesn’t really affect hard drive performance these days is the interface. Even 10,000RPM drives can’t fully saturate a 3gbps SATA I port. Seagate has suggested that the data coming off the 64MB cache of their latest second generation SATA 6gbps drive can saturate the bust, but this would be with brief bursts at best, and with no practical effect on performance.

Seagate was first to market with a 7,200RPM, 3TB hard drive, but your system BIOS needs to recognize it properly if you want to configure it as one large partition.

Western Digital and Seagate also make a line of “green” (low power consumption) drives. The WD GreenPower drives typically spin at 5,400RPM, while Seagate’s Barracuda Green drives usually run at 5,900RPM. Note that power usage while actually under heavy load isn’t that much lower, but these drives also typically offer sophisticated sleep modes which use very little power at idle. This type of technology is gradually being migrated to higher performance drives as well.

SSD Tech

Solid State Drives are still in their infancy as a technology. Products continue to evolve, and performance increases over time. This is particularly true of random write performance. First generation SSDs were hobbled by extremely slow random write times – often much slower than old fashioned rotating platters. That’s changed with newer generation of controllers, better garbage collection (which reorganizes the data to minimize the number of small, empty blocks) and trim command support with modern operating systems, in which the OS tells the SSD which blocks of data are considered deleted.

The cost per bit of SSDs is much higher than hard drives, and given the limitations of the manufacturing processes, the cost per bit will remain high, albeit declining gradually. Currently, 25nm flash memory parts are pretty much the order of the day, with 20nm on the near horizon. As Anand Shimpi noted in a recent article, the costs of flash chips prevent prices from getting lower. And the cutthroat competition means products get shipped that aren’t fully baked.

Still, we’ve been using an SSD RAID array in our primary system for both everyday use and gaming, and it would be tough to go back. The incredibly short boot times and fast application loading are seductive, and the thought of waiting for stuff to load is painful. Most users can’t afford large capacity SSDs or SSD RAID arrays, so modest size drives are pretty much the rule of the day. That’s one reason you see so many 120GB drives – it’s right in the sweet spot for pricing.

As with hard drives, there are a number of factors that drive performance:

• The type of flash. Drives using SLC (single-level cell) flash are faster than those built with MLC (multi-level cell), but lower density, so drives built with SLC flash memory are pricier. However, SLC based drives are not only higher performing, but last longer and consume less power. That’s why SLC drives are often used in server applications.
• The interface. Unlike rotating platter drives, newer SSDs can saturate a 3gbps interface. That’s why many are moving to the newer 6gbps SATA spec.
• The controller. The controller built into the drive itself is the real secret sauce. Note that legacy hard drives also have controllers, but controllers in SSDs have a far larger impact on performance. The current darling in the SSD controller space is Sandforce, with its SF-2281. But Intel controllers are pretty good. It’s also worth noting that OCZ bought Indilinx, a relatively new controller company. So the controller wars will likely go on.
• The firmware. The real issue with SSDs is that they’re pretty new technology. What does that mean in practical term? Bugs. If you cruise various online forums, you’ll find that SSDs often have oddball issues, like blue-screens, sudden loss of capacity and more.

Before you get too mired in all the details of controllers and flash memory types, remember that any good, current generation SSD will offer performance that’s nothing short of amazing, if you’re coming from a rotating platter drive. After using your PC with your shiny new SSD installed, you’ll find yourself expecting PCs to be that responsive – and wondering why that shiny new laptop your spouse just bought seems so damned slow. Hint: it’s not the CPU or memory.

A 120GB SSD like this Patriot Wildfire is an great performance boost for a laptop up to a couple of years old and running Windows 7.

The other important consideration to weigh when thinking about SSDs is capacity. As noted, the sweet spot right now is 120GB drives, which range in price from $170 to $300 for drives built in standard, 2.5-inch form factors. Newer 240GB drives cost well north of $300 to over $500. Want a 500GB SSD? Be prepared to shell out nearly $800 or more. So consider your budget and applications before taking the expensive step to SSDs. We’ll discuss some scenarios shortly.

Now that we have some basic understanding of hard drives and SSDs, let’s look at a few typical user scenarios.