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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GlItCh017 - Wednesday, March 18, 2009 - link
I just wanted to comment that the backstory portion to this article is simply the most interesting part to an article (or almost even an article inside the main article). On top of that, it is easily the most interesting article I have ever read simply because of that section. Really really must say that I enjoyed reading it!radguy - Wednesday, March 18, 2009 - link
I have been waiting for this one for a while and it was very informative. Thank you very much for it. I did pick up one of the patriot warp drives for my netbook. I was really happy until I installed avg free. So not running an antivirus on it anymore but I have drive image backup incase it goes bad. Overall pretty happy as it was only 80 bucks if I get my mir.I think I'm going to wait until windows 7 till I upgrade my primary desktop. 2 of those vertexs in raid 0 would be sweet though.
sleepeeg3 - Wednesday, March 18, 2009 - link
They were one of the first SSDs you reviewed and they use their own controller. How does their random write performance compare to everything else out now?These reviews made me totally reassess the purchase of the two Samsungs I bought. I had no idea the random writes on the Samsung drives were so bad. Other reviews show the Samsung drives doing better or at least near the X25-M in write tests: http://www.techreport.com/articles.x/15433/6">http://www.techreport.com/articles.x/15433/6 However, those tests probably would have been somewhat sequential.
nubie - Wednesday, March 18, 2009 - link
Grammatically awkward sentence on Page 21:"so if you own one of these drives - you owned a fixed version."
The tense is incorrect (own/owned). I think "own a fixed version" is still awkward, perhaps "you have the fixed version", also the "so" may be superfluous. You can replace the ", so if" with a "; if". Here is how I might re-write the sentence:
"The old firmware never shipped thanks to OCZ's quick acting; if you own one of these drives - you have a fixed version."
(I am not an expert, so feel free to correct me if I am wrong.)
Awesome article btw, thanks for setting me straight on SSD, I have been steering clear of them. I hope soon you can review SSD's and most are good to excellent. :)
Flyboy27 - Wednesday, March 18, 2009 - link
This article has answered every question I've had regarding SSDs recently. Thanks Anand!Flyboy27 - Wednesday, March 18, 2009 - link
If a 120gb Vertex was around $250 I would get one yesterday. I suppose I can wait though.7Enigma - Thursday, March 19, 2009 - link
For me, 2 60's or 2 80's for around that price and I'm sold. Want the Raid0.kgwagner - Wednesday, March 18, 2009 - link
I almost didn't read this article, as everybody and their brother seems to want to explain SSDs these days and most of the articles aren't much more than glorified press releases. But, this one truly took the drives to task and presented some valid information and explanations about the state of the art and where it needs to go. Kudos, Anand. Awesome show. Good job.Mr Perfect - Wednesday, March 18, 2009 - link
"Needless to say, there was some definite fallout from that review. I’m used to negative manufacturer response after a GPU review, but I’m always a bit surprised when it happens in any other segment."Obviously you can't make a business out of irritating manufacturers, but when there really are issues, the readers want to know about them. After all, that's why we come here!
gwolfman - Wednesday, March 18, 2009 - link
You own Anand. Keep up the good work. I've seen you cited from many sites about the work you've done, in particular with SSDs. Best article I've read in months!