The plain, unvarnished truth about the future of storage is that fundamental differences between hard-disk drives (HDDs) and solid-state drives (SSDs) will lead to the HDD’s doom. Those differences, however, also explain SSD’s bad reputation for getting slower over time, among other things. Such kinks are being worked out, and 2011 will be the year SSDs go mainstream.

To the SSD’s advantage, the HDD’s ancient spinning-disk tech makes the average latency (the time it takes to find the data) of a speedy Seagate Barracuda 7200.11 HDD a full 4.16 milliseconds. For a bits-on-chips Micron RealSSD C300, average write latency is a little over 0.08 milliseconds. Ouch. Also, a hard drive reads and writes data in a single stream, while an SSD deals simultaneously with multiple chip channels (four to ten, usually). The more channels, the higher the bandwidth. A serial HDD just can’t compete with a parallel SSD.

There are two types of SSDs. One, called a single-level cell (SLC), holds one bit per cell; the other, called a multi-level cell (MLC), holds two. The cells are essentially identical—it’s the way they store bits that’s different. But MLCs are cheaper than SLCs, and they’re (usually) slower and (potentially) have shorter lifespans. That’s why SSD manufacturers use a technique called wear-leveling to evenly balance cell usage. Essentially, it ensures that an SSD’s MLC cells grow old together.

The facts behind why older SSDs have slowed down relate to how bits are erased on HDDs and SSDs. When you delete a file, you don’t actually erase anything—you merely tell your Mac to forget about that file. When you need its space back on an HDD, you simply write over it. On an SSD, however, you have to erase it first, then write over it. But when you erase bits from an SSD, you have to do so one block at a time (usually 512KB), even though you write to one page at a time (there are commonly 128 4KB pages in a block).

Let’s say a block on your SSD contains data from multiple files. Some of those files are active, some are “deleted” but not erased. But now, when you want to write a new file to pages in that block, there are so many inactive—but full—pages that there’s no room for that file. So you need to erase those inactive pages.

But remember that you must erase the entire block at the same time. So to protect the active files in that block, you write the block out to some free space on your SSD or to a buffer, then erase the block, then write the original good stuff back to the just-erased block. And you’ve just wasted a ton of time.

What’s worse, the more you use your drive, the more it fills up, and the more time you waste keeping it clean. The older it gets, the slower it gets. The solution to this problem was once simply to wipe an aging, slowing SSD clean and start over—not convenient, but effective. But SSD designers have come up with better ideas, such as setting aside a good chunk of the SSD as extra space for write operations (called overprovisioning), then to erase inactive pages during garbage-collection idle time—and to have those overprovisioned blocks be available when others go south.

Another was the introduction of TRIM technology, in which the operating system explicitly tells the SSD to erase the pages it no longer needs. Windows 7 supports TRIM; Mac OS X doesn’t—though it’s rumored for the future.

More effective, however, are increasingly sophisticated, zippy drive controllers, such as the Indilinx Barefoot or the Marvell controller in the Micron RealSSD C300, that are smarter than TRIM. SandForce’s DuraClass Technology controllers, which power Other World Computing’s Mercury Extreme Pro SSDs, among others, add compression and other goodies to the mix.

So SSDs will get better. Indilinx’s new JetStream controller is coming soon, and Intel’s popular X25-M will update late in the year. It may take a generation for SSDs and HDDs to reach price parity, but when they get even close, hard-disk drives are doomed.

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Since the late 1980s, Rik Myslewski has paid his rent by keeping an eye on Apple. He was editor-in-chief of MacAddict from 2001 until its transformation into Mac|Life in early 2007, and is now a member of the snarkily sophisticated team at London’s The Register, which is “biting the hand that feeds IT” daily at www.theregister.co.uk.