The Instruction That Changes (almost) Everything: TRIM

TRIM is an interesting command. It lets the SSD prioritize blocks for cleaning. In the example I used before, a block is cleaned only when the drive runs out of places to write things and has to dip into its spare area. With TRIM, if you delete a file, the OS sends a TRIM command to the drive along with the associated LBAs that are no longer needed. The TRIM command tells the drive that it can schedule those blocks for cleaning and add them to the pool of replacement blocks.

A used SSD will only have its spare area to use as a scratch pad for moving data around; on most consumer drives that’s around 7%. Take a look at this graph from a study IBM did on SSD performance:


Write Amplification vs. Spare Area, courtesy of IBM Zurich Research Laboratory

Note how dramatically write amplification goes down when you increase the percentage of spare area the drive has. In order to get down to a write amplification factor of 1 our spare area needs to be somewhere in the 10 - 30% range, depending on how much of the data on our drive is static.

Remember our pool of replacement blocks? This graph actually assumes that we have multiple pools of replacement blocks. One for frequently changing data (e.g. file tables, pagefile, other random writes) and one for static data (e.g. installed applications, data). If the SSD controller only implements a single pool of replacement blocks, the spare area requirements are much higher:


Write Amplification vs. Spare Area, courtesy of IBM Zurich Research Laboratory

We’re looking at a minimum of 30% spare area for this simpler algorithm. Some models don’t even drop down to 1.0x write amplification.

But remember, today’s consumer drives only ship with roughly 6 - 7% spare area on them. That’s under the 10% minimum even from our more sophisticated controller example. By comparison, the enterprise SSDs like Intel’s X25-E ship with more spare area - in this case 20%.

What TRIM does is help give well architected controllers like that in the X25-M more spare area. Space you’re not using on the drive, space that has been TRIMed, can now be used in the pool of replacement blocks. And as IBM’s study shows, that can go a long way to improving performance depending on your workload.

Why Does My 80GB Drive Appear as 74.5GB? Understanding Spare Area Tying it All Together: SSD Performance Degradation
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  • Anand Lal Shimpi - Monday, August 31, 2009 - link

    Intel insists it's not an artificial cap and I tend to believe the source that fed me that information.

    That being said, if it's not an artificial cap it's either:

    1) Designed that way and can't be changed without a new controller
    2) A bug and can be fixed with firmware
    3) A bug and can't be fixed without a new controller

    Or some combination of those items. We'll see :)

    Take care,
    Anand
    Reply
  • Adul - Monday, August 31, 2009 - link

    Another fine article anand :). Keep up the good work. Reply
  • CurseTheSky - Monday, August 31, 2009 - link

    This is absolutely the best article I've read in a very long time - not just from Anandtech - from anywhere.

    I've been collecting information and comparing benchmarks / testimonials for over a month, trying to help myself decide between Intel, Indilinx, and Samsung-based drives. While it was easy to see that one of the three trails the pack, it was difficult to decide if the Intel G2 or Indilinx drives were the best bang for the buck.

    This article made it all apparent: The Intel G2 drives have better random read / write performance, but worse sequential write performance. Regardless, both drives are perfectly acceptable for every day use, and the real world difference would be hardly noticeable. Now if only the Intel drives would come back in stock, close to MSRP.

    Thank you for taking the time to write the article.
    Reply
  • deputc26 - Monday, August 31, 2009 - link

    been waiting months for this one. Reply
  • therealnickdanger - Monday, August 31, 2009 - link

    Ditto! Thanks Anand! Now the big question... Intel G2 or Vertex Turbo? :) It's nice to have options! Reply
  • Hank Scorpion - Monday, August 31, 2009 - link

    Anand,

    YOU ARE A LEGEND!!! go and get some good sleep, thanks for answering and allaying my fears... i appreciate all your hard work!!!!

    256GB OCZ Vertex is on the top of my list as soon as a validated Windows 7 TRIM firmware that doesnt need any work by me is organized....

    once a firmware is organised then my new machine is born.... MUHAHAHAHAHAHA
    Reply
  • AbRASiON - Monday, August 31, 2009 - link

    Vertex Turbo is a complete rip off, Anand clearly held back saying it from offending the guy at OCZ.
    Now the other OCZ models however, could be a different story.
    Reply
  • MikeZZZZ - Monday, August 31, 2009 - link

    I too love my Vertex. Running these things in RAID0 will blow your mind. I'm just waiting for some affordable enterprise-class drives for our servers.

    Mike
    http://solidstatedrivehome.com">http://solidstatedrivehome.com
    Reply
  • JPS - Monday, August 31, 2009 - link

    I loved the first draft of the Anthology and this is a great follow-up. I have been running a Vertex in workstation and laptop for months know and continue to be amazed at the difference when I boot up a comparable system still running standard HDDs. Reply
  • gigahertz20 - Monday, August 31, 2009 - link

    Another great article from Anand, now where can I get my Intel X-25M G2 :) Reply

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