The SSD Anthology: Understanding SSDs and New Drives from OCZ
by Anand Lal Shimpi on March 18, 2009 12:00 AM EST- Posted in
- Storage
The Anatomy of an SSD
Let’s meet Mr. N-channel MOSFET again:
Say Hello
This is the building block of NAND-flash; one transistor is required per cell. A single NAND-flash cell can either store one or two bits of data. If it stores one, then it’s called a Single Level Cell (SLC) flash and if it stores two then it’s a Multi Level Cell (MLC) flash. Both are physically made the same way; in fact there’s nothing that separates MLC from SLC flash, it’s just a matter of how the data is stored in and read from the cell.
SLC flash (left) vs. MLC flash (right)
Flash is read from and written to in a guess-and-test fashion. You apply a voltage to the cell and check to see how it responds. You keep increasing the voltage until you get a result.
| SLC NAND flash | MLC NAND flash | |
| Random Read | 25 µs | 50 µs |
| Erase | 2ms per block | 2ms per block |
| Programming | 250 µs | 900 µs |
With four voltage levels to check, MLC flash takes around 3x longer to write to as SLC. On the flip side you get twice the capacity at the same cost. Because of this distinction, and the fact that even MLC flash is more than fast enough for a SSD, you’ll only see MLC used for desktop SSDs while SLC is used for enterprise level server SSDs.
Cells are strung together in arrays as depicted in the image to the right
So a single cell stores either one or two bits of data, but where do we go from there? Groups of cells are organized into pages, the smallest structure that’s readable/writable in a SSD. Today 4KB pages are standard on SSDs.
Pages are grouped together into blocks; today it’s common to have 128 pages in a block (512KB in a block). A block is the smallest structure that can be erased in a NAND-flash device. So while you can read from and write to a page, you can only erase a block (128 pages at a time). This is where many of the SSD’s problems stem from, I’ll repeat this again later because it’s one of the most important parts of understanding SSDs.
Arrays of cells are grouped into a page, arrays of pages are grouped into blocks
Blocks are then grouped into planes, and you’ll find multiple planes on a single NAND-flash die.
The combining doesn’t stop there; you can usually find either one, two or four die per package. While you’ll see a single NAND-flash IC, there may actually be two or four die in that package. You can also stack multiple ICs on top of each other to minimize board real estate usage.
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Jamor - Wednesday, March 18, 2009 - link
The best tech article I've ever read, and I've read a few.haze4peace - Wednesday, March 18, 2009 - link
Wow, excellent article and so much useful information in an easy to understand way. I have just recently been paying attention to SSDs and thanks to this article I am armed with the information to make the correct choice for my needs. Thanks AnandTech, its the deep and honest articles like these that keep me coming back for more.Alseki - Wednesday, March 18, 2009 - link
I just registered then simply to say, great article. Really informative and enjoyable to read.alexsch8 - Wednesday, March 18, 2009 - link
Anand,Thank you for this article, very informative.
Looking at the example you are giving with your self-manufactured SSD drive: If I save the DOC I use up a page. Based on what you are saying, if I make a change to that DOC, it would then be saved in the next page instead of overwriting the existing page? If that is true, then the File Allocation system (FAT or MFT) itself would contribute quite a bit to the 'filling up of pages' phenomena. Could you elaborate if the proposed file system for SSD addresses this?
Ytterbium - Wednesday, March 18, 2009 - link
Fantastic article, shame that the vendors blacklisted you for telling the truth and OCZ rock for working so hard to address issues.I'll be ordering my Intel SSD soon, I'll defintly consider the Summit when it comes out for my encoding rig as there sequental writes matter to me.
mindless1 - Wednesday, March 18, 2009 - link
Great even, but I've have to disagree with the significance of the passage that suggested the Indilinx controller makes data loss as bad on those SSD as on a conventional hard drive.The primary cause of data loss is mechanical or component failure, not power loss. If we want to consider power loss, it's not just the drive which is prone to lose data, the entire system memory suffers far more data loss than that.
Further, a sufficiently sized supercapacitor should keep the drive operating for a period of time beyond when the rest of the system would be operational, it could be sufficient for the controller to finish writing to flash all received data (or just use an UPS, that's what they're for?).
Second, I can't believe that OCZ only tests designs with HDTach and Atto, I think it more likely they knew of the problem but didn't expect anyone to find it so quickly, and felt the higher sequential speeds made it more marketable. This makes me feel that manufacturers, then online sellers should differentiate their drives with a standardized random read/write score.
What would be really nice is if the Indilinx based SSDs had an application available, similar to a HDD acoustic management bit changing app, that lets the owner set their own preference for IO versus sequential read performance.
gomakeit - Wednesday, March 18, 2009 - link
This is by far the BEST article on SSD I've ever read! Great job anand and yes I read every single word of it!MagicPants - Wednesday, March 18, 2009 - link
Don't they ever try using their own devices? One second of latency should slap any user in the face. It should be very easy for a manufacturer to build a system with their new technology put it in front of people and see what happens, but apparently they're not doing this.They wait for reviewers to do the work for them and then get upset when they find a problem.
What the manufacturers should be taking away from this article is:
1) Try your competitor's products
2) Try your own products
3) Try them in real life as opposed to synthetic tests
4) Compare everything you've tried and market the performance that matters
7Enigma - Thursday, March 19, 2009 - link
But that would make sense....and we know marketing rarely does.paulinus - Wednesday, March 18, 2009 - link
That art is great. Finally someone done ssd test's right, and said loud what we, customers, can get for that hefty pricetags.I've supposed that only choices are intel and new ocz's. Now I know, and big kudos for that.
Just need a bit more $$ for x25-m, it'll be ideal for heavy workstation use, and biggest vertex'll replace wd black in my aging 6910p :)