Understanding solid-state drives
It seems I've forgotten about writing for a few months. I miss writing to be honest and I think it's time to end the hiatus by writing a little something about a package I received a few weeks ago.
It seems the Intel X25-M solid-state drive has been turning a lot of heads. I'm not sure very many people really understand the implications of both Intel's advances and solid-state technology in general. They look at the benchmarks and the two orders of magnitude lower seek time. They smile and nod and that's it…end of story. Not so fast.
SLC versus MLC Solid-state drives are usually classified into two categories: Those made from single-level cells (SLC) and those made from multi-level cells (MLC). The difference is SLC stores 1-bit per transistor and MLC stores 2-bits per transistor. The implications are realized in performance, reliability and cost.
Performance-wise MLC typically takes twice as long to read, the same amount of time to erase, and usually more than three times longer to write than SLC. Keep in mind that athough MLC is significantly slower to write to than SLC it's still faster than a rotational disk.
SLC's biggest advantage is lifespan not performance: SLC cells can be erased and reprogrammed 100,000 times versus 10,000 for MLC. The reason this difference is important is write amplification. Write amplification is the actual size of the write versus the requested write size; This occurs when saving a file that is smaller than the page/block size on an SSD. SSDs have traditionally had a write amplification of 20-40x.
Price-wise it's much more expensive to build SLC SSDs over MLC because you need twice the number of transistors to store the same amount of data. If you thought MLC was out of your budget SLC is way out of your league.
AnandTech does a great job of explaining the technical details of how NAND flash memory works if you're even more curious.
What makes the Intel X-25M better than other SSDs?
1. Reduce write amplication The Intel drive has reduced the write amplification to 1.1 or less which is hugely impressive. This contributes to massively improving the lifespan of the device.
2. Build a better controller Intel's excellent implementation of a controller makes a pretty big difference. Other SSDs have done a really weak job in this aspect. Intel nails it with their controller. It improves caching, queuing and running concurrent operations, improves the lifespan and provides additional information specific to SSDs.
3. Better wear-leveling The Intel controller takes advantage of the fact that read performance is the same anywhere on the disk, so deleting a file, and redownloading it will write it to another part of the SSD; This is called wear leveling. Intel's wear leveling has a difference of 1.1 or less between maximum wear and average wear versus the traditional difference of ~3x. This also helps improve drastically the lifespan of the device.
4. Revolutionary improvements to reliability Intel promises you can write 100GB per day for 5 years before the drive fails but it should last much longer than that. Intel does that by keeping 7.5-8% (6-6.4GB on the 80GB drive) in reserve so that as blocks fail it can switch the reserve blocks. The drive supports two additional SMART attributes, one that says how close to the rated cycling limit you are, and one that says when you've run out of (or are about to run out) reserve blocks. This means you'll know exactly when your drive is about to fail. That's frickin' awesome!
Results These are my Xbench disk results before and after on my unibody MacBook 2GHz with 4GB of RAM. Real-world increase of 5.1x in disk performance. Everything just feels…instant. Pinwheels are an extremely rare occurrence.
I can't even imagine the scary performance Intel can wring out of their upcoming SLC drive. It's going to be a monster, and a game changer quite possibly (in server environments).
What Intel has done is raise the bar for everyone by showing them what's really possible with solid-state drives. Honestly after weeks of use and comparing this to my daily use of a faster MacBook Pro without an SSD at work, I can say it's the single best upgrade you can do to any computer to make your computing life better…if you can afford one. It's a night and day difference.
The best part is the drive's already dropped over $100 since I bought it; It's $539 $399 on Amazon.
