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Thought I would pass this on: All About Solid State Drives It's a new season in the storage device game. The familiar mechanism-free approach of our flash memory cards and thumb drives has made it to the big leagues where hard drives play ball. But can the Solid State Drive rookie phenom handle big league pitching? SSDs looked pretty good in spring training, showing off some very attractive qualities: no moving parts, the fastest read speeds possible, lower power consumption and -- not least -- tolerance for heat and vibration. Perfect for the laptop circuit, you might think. But their weakness at the plate has been worrisome.
The problem with SSDs is their finite storage life, known as write endurance. You can only write to them a certain number of times before they ignore you. It's a lot of times (roughly between 20,000 to 1,000,000 write/erase cycles depending on the type of flash memory cell used) but for the kind of nearly constant writing a hard disk in a computer needs to perform, it's an issue that needs special engineering. On a conventional hard disk, data is written to any free sector and read back from that sector on demand. If the data in the sector is changed, the hard drive controller just overwrites that sector, assigning a new Cyclic Redundancy Check value at the same time. Some sectors are used repeatedly, others never get touched -- and that's no threat to your data. But on an SSD, changed data is written to a just-erased block without touching the block that has the original data. That's one way to even out the use of the drive. But the SSD controller has to keep tabs on which blocks are current. So the controller keeps track of how many times each block has been used and reuses those that have been used the least before reusing the others. That's called wear leveling and even your flash card does a little of that. But that's where the similarity stops. The SSD controller has a lot more work to do than a flash card. It keeps track of how often a block is accessed and if it's not often, it moves it to a more frequently used block to give that block a rest. It's constantly trying to even out the use of the drive independently of the operating system's write requests. That static wear leveling ensures you won't have an unused driver or rarely-touched file sitting on your SSD that, when you finally try to access it, is corrupted. It will be continually refreshed so it's readable when you need it. While performance is spectacular on a new drive, as you use most of them, it seriously degrades. It takes longer and longer for the controller to write to the SSD. To recover performance, laborious reconditioning is required. One day that may be automatic, but at the moment, you have to do it yourself. Windows has the TRIM command and there are hardware-specific wiper utilities for drives running other operating systems.
To find out just how ready SSDs are for the big leagues, we cornered a Silicon Valley batting coach who asked to remain nameless.
Q&A
Q. Why would anyone want to use an SSD?
A. Performance. That's it in a sunflower seed shell. Nothing reads faster. You've got very quick start-ups and your applications load instantly. You don't wait for a platter to spin the data under a head for you. It's just there. Of course, it costs a lot more per gigabyte. But if you have to fly, you have to fly.
Q. So it's a lot like a thumbdrive, only bigger?
A. Not really. Thumbdrives -- even the flash cards you use in your camera -- don't work as hard as a hard drive. You write to them very seldom compared to a hard drive. You read from them a lot. But reading doesn't hurt them.
Q. What does hurt them?
A. Erasing -- which you have to do before a write -- and writing. That's what wears them out. So the controller has to be careful to write efficiently to the SDD to maintain a reasonable life expectancy. One way it does that is by writing in small pages to a larger block the controller knows has already been erased, so every write doesn't need an erase. But there's a limit to the number of writes you can make. A generous limit, but a limit.
Q. Which is what?
A. Well, it depends. For a typical laptop user, say, a sophisticated controller can stretch the lifetime of an SSD to maybe four or five years. A bit longer than the life of the laptop. In fact, I've never seen an SSD come back because it's worn out. Even after a years in the field. So you might put it this way: with an SSD, you won't have to worry about replacing the hard drive before you replace the laptop. It won't crash.
Q. Sounds good to me. So what's the problem?
A. Besides write endurance -- how often you can write to it -- there's the issue of data retention or how long an SSD holds the data after you remove the power. You really wouldn't use an SSD for archiving. After about a year the data on a heavily used drive needs to be refreshed. But you can get 10 years data retention right out of the box on an unused drive. So it depends how much the drive has been used.
Q. So an SSD doesn't store data like a hard drive?
A. No, it uses these NAND floating gate transistors that maintain a certain state not a particular value. So you have to erase a block before you can write to it. You have to establish one state (say a logical zero) before you can apply voltage to change it to another state (say a logical one).
Q. Really?
A. Well, this is where it gets interesting. Write endurance and data retention go together. The drive manufacturer can spec a higher endurance if the user can live with lower data retention. And, just the opposite, lower endurance can buy higher data retention. Depends.
Q. So why you have to do reconditioning?
A. Say you have block sizes of 512K and, like any hard drive, you write to full blocks. Well, the data you write could be 4K or 8K or just 16K, so you aren't using the whole 512K. There's always a mismatch between block size and what you write to the block. So you end up with a lot of unused capacity. Maybe half the drive is actually unused. You have to do get that space back. The controller always tries to write efficiently, storing the writes in its DRAM buffer until it can write them sequentially to the drive (rather than randomly as the computer has requested). Then it just maintains pointers to the logical blocks in DRAM. That minimizes the number of erases and writes. And it even uses some scratchpad space on the drive -- which we call over-provisioning -- to organize things.
But over time, as the drive fills up, it has a tough time finding this scratchpad space. Up to 72 percent full, write performance is the same, but between 72 and 93 percent full, it drops off dramatically. So, with reconditioning, you consolidate the data and free up all those little empty areas, putting them back in harness. It's just garbage collection.
Q. How much over-provisioning do you need?
A. Well, you know, you buy an SSD in a binary capacity of, say, 64-GB. That's 64 times 1024 times 1024 times 1024. But the drive is sold in billions of bytes calculated as 64 times 1000 times 1000 times 1000. The difference nets out to seven percent over-provisioning. You don't have access to that space (and don't know it).
Buy a 60-GB drive and you have 13 percent over-provisioning. Because you're really buying a 64-GB drive of which you can access only 60-GB. And a 480-GB drive is really a 512-GB drive with 13 percent over-provisioning. But a 50-GB drive or a 400-GB drive just has more over-provisioning: 27 percent.
The more over-provisioning you have, the less often your drive has to recondition itself because it prevents the drive from filling up. You'll notice that magical 72 percent full mark, under which write performance stays high is the same as 28 percent of the disk free, which is what 27 percent over-provisioning is. So performance on a disk with 27 percent over-provisioning -- and a smart controller -- doesn't degrade.
Q. So what should I look for in an SSD?
A. Besides price, you want to look at capacity. It not only tells you how much over-provisioning you have but larger drives are faster because they write in parallel. A 128/256-GB class drive is actually about twice as fast as a 32/64-GB drive. That's a lot different than a conventional hard drive, whose performance is constant despite its size because it depends on buffer size and spindle speed.
Q. What's next in SSD technology?
A. We're going to see continued write performance improvement, although it will still be limited by the SATA connection in the computer. We'll also see advances in how we monitor the health of the hard drive. And, maybe most exciting, there's really no requirement that SSDs be shaped like hard drives. We may start seeing SSD-designed machines that are thinner and smaller than our tablets, laptops and other devices today.
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