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
Strength in Numbers, What makes SSDs Fast
Given the way a single NAND-flash IC is organized one thing should come to mind: parallelism.
Fundamentally the flash that’s used in SSDs cut from the same cloth as the flash that’s used in USB drives. And if you’ve ever used a USB flash drive you know that those things aren’t all that fast. Peak performance to a single NAND-flash IC is going to be somewhere in the 5 - 40MB/s range. You get the faster transfer rates by reading/writing in parallel to multiple die in the same package.
The real performance comes from accessing multiple NAND ICs concurrently. If each device can give you 20MB/s of bandwidth and you’ve got 10 devices you can access at the same time, that’s 200MB/s of bandwidth. While hard drives like reads/writes to be at the same place on the drive, SSDs don’t mind; some are even architected to prefer that data be spread out all over the drive so it can hit as many flash devices as possible in tandem. Most drives these days have 4 - 10 channel controllers.
The Recap
I told you I’d mention this again because it’s hugely important, so here it is:
A single NAND flash die is subdivided into blocks. The typical case these days is that each block is 512KB in size. Each block is further subdivided into pages, with the typical page size these days being 4KB.
Now you can read and write to individual pages, so long as they are empty. However once a page has been written, it can’t be overwritten, it must be erased first before you can write to it again. And therein lies the problem, the smallest structure you can erase in a NAND flash device today is a block. Once more, you can read/write 4KB at a time, but you can only erase 512KB at a time.
It gets worse. Every time you erase a block, you reduce the lifespan of the flash. Standard MLC NAND flash can only be erased 10,000 times before it goes bad and stops storing data.
Based on what I’ve just told you there are two things you don’t want to do when writing to flash: 1) you don’t want to overwrite data, and 2) you don’t want to erase data. If flash were used as a replacement for DVD-Rs then we wouldn’t have a problem, but it’s being used as a replacement for conventional HDDs. Who thought that would be a good idea?
It turns out that the benefits are more than worth the inconvenience of dealing with these pesky rules; so we work around them.
Most people don’t fill up their drives, so SSD controller makers get around the problem by writing to every page on the drive before ever erasing a single block.
If you go about using all available pages to write to and never erasing anything from the drive, you’ll eventually run out of available pages. I’m sure there’s a fossil fuel analogy somewhere in there. While your drive won’t technically be full (you may have been diligently deleting files along the way and only using a fraction of your drive’s capacity), eventually every single block on your drive will be full of both valid and invalid pages.
In other words, even if you’re using only 60% of your drive, chances are that 100% of your drive will get written to simply by day to day creation/deletion of files.
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Luddite - Friday, March 20, 2009 - link
So even with the TRIM command, when working with large files, say, in photoshop and saving multiple layers, the performance will stil drop off?proviewIT - Thursday, March 19, 2009 - link
I bought a Vertex 120GB and it is NOT working on my Nvidia chipsets motherboard. Anyone met the same problem? I tried intel chipsets motherboard and seems ok.I used HDtach to test the read/write performance 4 days ago, wow, it was amazing. 160MB/s in write. But today I felt it slower and used HDtach to test again, it downs to single digit MB per second. Can I recover it or I need to return it?
kmmatney - Thursday, March 19, 2009 - link
Based on the results and price, I would say that the OCZ Vertex deserves a Editor's choice of some sort (Gold, Silver)...Tattered87 - Thursday, March 19, 2009 - link
While I must admit I skipped over some of the more technical bits where SSD was explained in detail, I read the summaries and I've gotta admit this article was extremely helpful. I've been wanting to get one of these for a long time now but they've seemed too infantile in technological terms to put such a hefty investment in, until now.After reading about OCZ's response to you and how they've stepped it up and are willing to cut unimportant statistics in favor of lower latencies, I actually decided to purchase one myself. Figured I might as well show my appreciation to OCZ by grabbing up a 60GB SSD, not to mention it looks like it's by far the best purchase I can make SSD-wise for $200.
Thanks for the awesome article, was a fun read, that's for sure.
bsoft16384 - Thursday, March 19, 2009 - link
Anand, I don't want to sound too negative in my comments. While I wouldn't call them unusable, there's no doubt that the random write performance of the JMicron SSDs sucks. I'm glad that you're actually running random I/O tests when so many other websites just run HDTune and call it a day.That X25-M for $340 is looking mighty tempting, though.
MrSpadge - Thursday, March 19, 2009 - link
Hi,first: great article, thanks to Anand and OCZ!
Something crossed my mind when I saw the firmware-based trade-off between random writes and sequential transfer rates: couldn't that be adjusted dynamically to get the best of both worlds? Default to the current behaviour but switch into something resembling te old one when extensive sequential transfers are detected?
Of course this neccesiates that the processor would be able to handle additional load and that the firmware changes don't involve permanent changes in the organization of the data.
Maybe the OCZ-Team already thought about this and maybe nobody's going to read this post, buried deep within the comments..
MrS
Per Hansson - Thursday, March 19, 2009 - link
Great work on the review AnandI really enjoyed reading it and learning from it
Will there be any tests of the old timers like Mtron etc?
tomoyo - Thursday, March 19, 2009 - link
That was kind of strange to me too. But I assume Anand really means the desktop market, not the server storage/business market. Since it's highly doubtful that the general consumer will spend many times as much money for 15k SAS drives.Gary Key - Thursday, March 19, 2009 - link
The intent was based it being the fastest for a consumer based desktop drive, the text has been updated to reflect that fact.tomoyo - Thursday, March 19, 2009 - link
I've always been someone who wants real clarify and truth to the information on the internet. That's a problem because probably 90% of things are not. But Anand is one man I feel a lot of trust for because of great and complete articles such as this. This is truly the first time that I feel like I really understand what goes into ssd performance and why it can be good or bad. Thank you so much for being the most inciteful voice in the hardware community. And keep fighting those damn manufacturers who are scared of the facts getting in the way of their 200MB/s marketing bs.