Understanding Spare Area (or Why My 80GB Drive Appears as 74.5GB)

Intel's 80GB X25-M has 80GB of NAND flash on it. That's 85,899,345,920 bytes or 80 x 1024^3 bytes (1024 bytes in a kilobyte x 1024 kilobytes in a megabyte x 1024 megabytes in a gigabyte x 80 gigabytes).

Hard drive makers however assume that 80GB means 80,000,000,000 bytes, since they use the definition of 1GB = 1,000,000,000 bytes. SSD vendors thus use the same definition. Now 80,000,000,000 bytes actually equals 74.5GB, so that's all the space you get to use on the drive.

How much space is there really on the drive? 80GB. How much space does Windows let you use? 74.5GB. What happens to the remaining 5.5GB? It's used by the drive's controller as spare area.

Intel’s controller is dynamic, it uses the entire drive as spare area until you’ve written every LBA once. Then it relies on the remaining 7.5% of non-user-space as its scratch pad. That’s why its new, out of box, performance is so good.

Other controllers may not be quite as dynamic, but they may also take a smaller performance hit when fully used. Why would Intel work so hard to make its out of box performance so high, even when it’ll be short lived? Because of TRIM.

A Wear Leveling Refresher: How Long Will My SSD Last? The Instruction That Changes (almost) Everything: TRIM
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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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