Friday, October 23, 2009

The (solid) state of hard-drive storage

What part of your system has more moving bits than any other?

What's the most unreliable yet valuable part of your computer system?

What part of your system is probably the biggest bottle-neck to the overal performance of your computer?

Most people will realise that I'm talking about the hard-drive.

The basic design of the hard drive hasn't changed for decades. It consists of one or more metal disks that spin around at a great number of RPMs (over 10,000 in more recent hi-performance drives) and a number of tiny heads that float less than a hair's breadth above those platters in a way that represents a disaster just waiting to happen.

Imagine a modern jet-fighter flying at the speed of sound just a couple of metres above the ground - well if you were to scale up the internals of your hard-drive, that's the kind of action we're talking about.

And, like any device that contains moving parts, your hard drive will eventually wear out -- that's if the heat generated by its electronics and motor don't kill it first.

This, of course, is why we make backups and use RAID arrays when storing important data.

However, the days of the now ancient mechanical hard-drive are drawing to a close and already we're seeing a family of (netbook) computers that ship without them, yet offer the ability to store gigabytes of programs and data.

Enter the age of the Solid State Drive (SSD).

Thanks to rapid advances in non-volatile memory technology, it's now possible to create solid state storage systems that emulate a regular SATA hard drive -- but without all the vulnerable mechanics or moving parts.

Think of them as an uber-USB drive -- but much bigger and faster.

SSDs have numerous advantages over conventional hard drives and those include far greater reliability, greatly improved access speeds, less vulnerability to unexpected power failures, potentially lower power consumption and faster read/write data rates.

These solid-state drives are also far less susceptible to the effects of physical shock or extremes of temperature and humidity. In effect, they're almost the perfect storage medium.

The only real downside at present is their somewhat higher price than mechanical drives -- but that will also change over time.

Already available in regular hard-drive form-factors and with the ubiquitous SATA electrical interface, it's now possible to perform "plug and play" replacements with almost any desktop or laptop computer, swapping the existing drive for an SSD equivalent of up to 1TB.

So why has it taken so long for SSDs to catch on? Apart from price, what's been holding them back?

Well part of that boils down to a small issue called write-cycle-life or "write endurance".

The problem with Flash memory (which most SSDs use) is that you can only write to it a fixed number of times before the computer equivalent of alzheimer's sets in. Once the write-cycle life of a Flash memory cell is exceeded, it can no longer reliably hold data and the write may either fail or random bit-flipping can occur.

Early Flash memory suffered from a write-cycle life that was measured in just thousands of write operations but this figure has slowly improved and now most good quality silicon has a life measured in millions of individual write-cycles.

Of course you might think that's still an awfully small number, when you work out how quickly computers like to flip bits and how often data gets written to a hard drive -- but things really aren't that bad.

That's because the makers of SSDs have implemented some clever schemes to mitigate the limited write-cycle life of the devices used.

For a start, they often use a level of indirection that ensures the write operations are spread out across the entire array of memory. This means that when you re-write a sector to the SSD, it may actually be written to a totally different part of the memory array to that it originally occupied. This is done so as to spread those write-cycles out and not excessively "wear" any given set of memory locations.

Some pretty sophisticated caching is also done using conventional RAM so that when a sector is constantly being re-written, it's not committed to actual Flash ram until the updates are completed or some other condition (such as a power failure) is signaled.

By use of these clever tactics, the effective write-cycle life of an SSD in regular use already exceeds the MTTF for a conventional hard drive my at least an order of magnitude.

Bearing all this in mind it becomes pretty obvious that the days of the existing whiny, spinning, vulnerable mechanical hard drive are numbered and that your next PC (or maybe the one after that) will almost certainly come fitted with a solid-state drive.

The arrival of the affordable SSD may do more for the effective power of the desktop computer over the next five or six years than enhancements in CPU architecture can contribute. The bottleneck and comparative unreliability of the slow hard-drive is about to be eliminated by some clever silicon.

Cheaper, faster, smaller -- when it comes to computers and storage its the way the future's shaping up.

Isn't progress a wonderful thing?

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