Wednesday, September 10, 2008

Not all SSDs are created Equal

Solid state disks (SSDs,) are the craze of the moment. These storage devices use Flash memory like the ones you can find in USB Pen drives and promisse to revolutionize the computer industry. However, there's also a dark side you should be aware of...

As anyone reading that has heard from it will be able to tell you, SSDs are awesome: no moving parts, nearly instantaneous acesses, the works!

That's the good part, however...

Not every SSD is created equal, and even if they use the exact same Flash memory chips, the device chipset and controller can make all the difference in the world. To make it worse, this is the kind of thing hidden away inside its case - not something you can tell by looking at its enclosure.

That's why we need to be careful when we see things like this SSD OCZ Core 32GB costing just $99 while others cost substantially more like this SSD from Intel costing $595.
Some people might be led to believe that such a huge price difference will more than cover the smaller size and slightly slower performance.

But things are not that easy...

Although Flash memory has some advantages, they also have one very annoying drawback: its write cycle times. (Not to mention the limited number of write cycles each cell can endure, but that's another issue.)

Simply put, when you write a very small 200 byte file to your SSD, internally it will need to erase a full "page" (which might be 256KB or even larger) which takes a very long time - relatively speaking.

You've probably noticed that already: when you copy  file to a USB Flash disk, it takes quite a bit longer to write to it than to read it back to the disk.

To mess things up even more, there are different flash types available, the most common used in SSDs being SLC and MLC.
MLCs are cheaper - and as you'd might imagine, slower. However, when combined with an intelligent controller even these "low-cost" flashs can beat SLC SSDs with "dumb" controllers.
(Intel MLC SSDs are a prime example of this, beating many SLC SSDs from other manufacturers.)

So, what does this all mean anyway?

It means that in some cases, SSDs like OCZ Core can become annoyingly slow and grind your computer to an halt for several seconds.
Check the worst case scenario below:

While an Intel SSD takes 0.09ms to perform a 4K write, an OCZ Core takes thousands of times more - worst case being nearly 1 second!

In real world this can mean you'll have to endure frequent system "hiccups" or even be unable to install Vista (besides taking a long time, it seems Vista doesn't handle well those lengthy write operations and gets corrupted somehow.)

Again, keep in mind I'm talking about the OCZ Core and similar SSDs - other SSDs like the one from Intel performs just like we expect an SSD to perform: flawlessly!

Of course, there are tasks where even a poorly designed SSD can perform extremely well; like you can see in this benchmark that is executing 4 tasks together: searching Windows contacts, searching Windows mails, browsing in IE7, and launching applications.

Even though slower than faster and better designed SSD, it still beats magnetic hard drives.

However, you have to remember that you might have to face those dreaded deadlocks for a few seconds every now and then. So, keep that in mind when you see some extra low-cost offering for an SSD.

If you want a much more in-depth review about SSDs and its internal workings, be sure to read:
[Detailed performance analysis at Anandtech]

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