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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strikeback03 - Thursday, March 19, 2009 - link
I understand your point, but I am not sure you understand the point I (and others) are trying to make. The SSD makers (should) know their market. As they seem to be marketing these SSDs to consumers, they should know that means the vast majority are on Vista or OSX, so the OS won't be optimized for SSDs. It also means the majority will be using integrated disk controllers. Therefore, in choosing a SSD controller which does not operate properly given those restrictions, they chose poorly. The testing here at Anandtech shows that regardless of how the drives might perform in ideal circumstances, they have noticeable issues when used the way most users would use them, which is really all those users care about.tshen83 - Thursday, March 19, 2009 - link
In the history of computing, it was always the case that software compensated for the new hardware, not the other way around. When new hardware comes out that obsoletes current generation of software, new software will be written to take advantage of the new hardware.Think of it this way: you always write newer version of drivers to drive the newest GPUs. When is the last time newer GPUs work with older drivers?
Nobody should be designing hardware now that makes DOS run fast right? All file systems (except ZFS and soon BTRFS) are obsolete now for SSDs, so we write new file systems. I am not sure Intel X25-M's approach of virtualizing flash to the likings of NTFS and EXT3 is the correct one. It is simply a bridge to get to the next solution.
SSD makers right now are making a serious mistake pushing SSDs down consumer's throats during an economic crisis. They should have focused on the enterprise market, targeting DB servers. But in that space, Intel X25-E sits alone without competition. (Supertalent UltraDrive LEs should be within 25% of X25-E by my estimation)
pmonti80 - Thursday, March 19, 2009 - link
Now I understand what you meant in the beginning. But still I don't agree with you, the system reviewed is the one 99% of SSD buyers will use(integrated mobo controller + NTFS). So, why optimize the benchmark to show the bad drives in a good light?About the Vertex, I don't understand what you are complaining about. After reading this article most people got the idea that Vertex is a good drive and at half Intel's price (I know, I searched on google for comments about this article).
tshen83 - Thursday, March 19, 2009 - link
Professional people only look at two SSD benchmarks: random read IOPS at 4k and random write IOPS at 4k(Maybe 8K too for native DB pages).The Vertex random write IOPS at 4K size is abysmal. 2.4MB/sec at 4K means it only does 600ish random write IOPs. Something was wrong, and Vista/ICH10R didn't help. The 8GB/sec write boundary Anand imposed on the random write IOPS test is fishy. So is the artificial io queue depth = 3.
The vertex random write IOPS should be better. The random read IOPS also should be slightly better. I have seen OCZ's own benchmark placing the Vertex very close to Intel X25-M at random read/ write IOPS tests.
I personally think that if you use NTFS, just ignore the SSDs for now until Windows 7 RTM. You can't hurt waiting for SSD price to drop some more in the next 6 months. Same thing for Linux, although I would argue that Linux is even in a worse position for SSDs right now than windows 7. EXT3/EXT4/JFS/XFS/REISERFS all suck on SSDs.
gss4w - Thursday, March 19, 2009 - link
Anandtech should adopt the same comment system as Dailytech so that comments that don't make any sense can be rated down. Who would want to read a review of something using a beta OS, or worse an OS that is only used on servers? I think it would be interesting to see if Windows 7 beta offered any improvements, but that should not be the focus of the review.7Enigma - Thursday, March 19, 2009 - link
Here's another vote for the Dailytech comments section. The ability to rate up down, but more importantly HIDE the comments below a threshold would make for much more enjoyable reading.curtisfong - Wednesday, March 18, 2009 - link
Why should Anand test with Windows 7b or *nix? What is the majority OS?Kudos to Anand for testing real world performance on an OS that most use, and to Intel for tuning their drives for it. I'm happy the other manufacturers are losing business..maybe they will also tune their drives for real world performance and not synthetic benchmarks.
To the poster above: do you work for OCZ or Samsung?
Glenn - Wednesday, March 18, 2009 - link
tshen83 "A very thorough review by tshen83, an hour agoBUT, still based on Windows Vista.
"
As long as these drives are marketed toward said OS, why would you not use it? Most of us wouldn't recognize Solaris if we saw it! And I believe you seriously overestimate yourself if your gonna drill anything into Anands head! You might need your own site, huh?
Great Job Anand! Don't forget to remind these CEO's that they also need to provide any software needed to configure and optimize these drives to work properly. ie go to OCZ Forums and try to figure out how to align, optimize and keep your drive running like it's supposed to, in less than 4 hours of reading! It would be nice if these companies would do their own beta testing and not rely on early adopters to do it for them!
Roland00 - Wednesday, March 18, 2009 - link
It was a joy to read all 31 pagesMagicPants - Wednesday, March 18, 2009 - link
Anand it would be really helpful to have a list of SSD companies blacklisting you so I know which ones to avoid. In general it would be nice to know who doesn't provide review samples to reputable sites.