Free Space to the Rescue

There’s not much we can do about the scenario I just described; you can’t erase individual pages, that’s the reality of NAND-flash. There are some things we can do to make it better though.

The most frequently used approach is to under provision the drive. Let’s say we only shipped our drive with 20KB of space to the end user, but we actually had 24KB of flash on the drive. The remaining 4KB could be used by our controller; how, you say?

In the scenario from the last page we had to write 12KB of data to our drive, but we only had 8KB in free pages and a 4KB invalid page. In order to write the 12KB we had to perform a read-modify-write which took over twice as long as a 12KB write should take.

If we had an extra 4KB of space our 12KB write from earlier could’ve proceeded without a problem. Take a look at how it would’ve worked:

We’d write 8KB to the user-facing flash, and then the remaining 4KB would get written to the overflow flash. Our write speed would still be 12KB/s and everything would be right in the world.

Now if we deleted and tried to write 4KB of data however, we’d run into the same problem again. We’re simply delaying the inevitable by shipping our drive with an extra 4KB of space.

The more spare-area we ship with, the longer our performance will remain at its peak level. But again, you have to pay the piper at some point.

Intel ships its X25-M with 7.5 - 8% more area than is actually reported to the OS. The more expensive enterprise version ships with the same amount of flash, but even more spare area. Random writes all over the drive are more likely in a server environment so Intel keeps more of the flash on the X25-E as spare area. You’re able to do this yourself if you own an X25-M; simply perform a secure erase and immediately partition the drive smaller than its actual capacity. The controller will use the unpartitioned space as spare area.

Understanding the SSD Performance Degradation Problem The Trim Command: Coming Soon to a Drive Near You
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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? Reply
  • 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?
    Reply
  • 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)... Reply
  • 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.
    Reply
  • 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.
    Reply
  • 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
    Reply
  • Per Hansson - Thursday, March 19, 2009 - link

    Great work on the review Anand
    I really enjoyed reading it and learning from it
    Will there be any tests of the old timers like Mtron etc?
    Reply
  • 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. Reply
  • 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. Reply
  • 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. Reply

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