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

    I believe OCZ cut prices to distributors that day, but the retail prices will take time to fall. Once you see X25-M G2s in stock then I'd expect to see the Indilinx drives fall in price. Resellers won't give you a break unless they have to :)

    Take care,
    Anand
    Reply
  • bobjones32 - Monday, August 31, 2009 - link

    Another great AnandTech article, thanks for the read.

    Just a head's-up on the 80GB X-25m Gen2 - A day before Newegg finally had them on sale, they bumped their price listing from $230 to $250. They sold at $250 for about 2 hours last Friday, went back out of stock until next week, and bumped the price again from $250 to $280.

    So....plain supply vs. demand is driving the price of the G2 roughly $50 higher than it was listed at a week ago. I have a feeling that if you wait a week or two, or shop around a bit, you'll easily find them selling elsewhere for the $230 price they were originally going for.
    Reply
  • AbRASiON - Monday, August 31, 2009 - link

    Correct, Newegg has gouged the 80gb from 229 to 279 and the 160gb from 449 to 499 :(

    Reply
  • Stan Zaske - Monday, August 31, 2009 - link

    Absolutely first rate article Anand and I thoroughly enjoyed reading it. Get some rest dude! LOL
    Reply
  • Jaramin - Monday, August 31, 2009 - link

    I'm wondering, if I were to use a low capacity SSD to install my OS on, but install my programs to a HDD for space reasons, just how much would that spoil the SSD advantage? All OS reads an writes would still be on the SSD, and the paging file would also be there. I'm very curious about the amount of degradation one would see relative to different use routines and apps. Reply
  • Anand Lal Shimpi - Monday, August 31, 2009 - link

    Putting all of your apps (especially frequently used ones) off of your SSD would defeat the purpose of an SSD. You'd be missing out on the ultra-fast app launch times.

    Pick a good SSD and you won't have to worry too much about performance degradation. As long as you don't stick it into a database server :)

    Take care,
    Anand
    Reply
  • swedishchef - Tuesday, September 01, 2009 - link

    What if you just put your photoshop cache on a pair of Velociraptors? Would it be the same loss of benefit?

    I have the same question regarding uncompressed HD video work, where I need write speeds well over the Intel x25-m ( over 240Mb/s). My assumption would be that I could enjoy the fast IO and App. launch of an SSD and increase CPU performance with the SSD while keeping the files on a fast external or internal raid configuration.


    Thank you again for a a brilliant Article Anand.
    I have been waiting for it for a long time. Yours are the only calm words out on the net.

    Grateful Geek /Also professional image creator.
    Reply
  • creathir - Monday, August 31, 2009 - link

    Great article Anand. I've been waiting for it...

    My only thoughts are, why can't Intel get their act together with the sequential business? Why can the others handle it, but they can't? To have such an awesome piece of hardware have such a nasty blemish is strange to me, especially on a Gen-2 product.

    I suppose there is some technical reason as to why, but it needs to be addressed.

    - Creathir
    Reply
  • Anand Lal Shimpi - Monday, August 31, 2009 - link

    If Intel would only let me do a deep dive on their controller I'd be able to tell you :) There's more I'd like to say but I can't yet unfortunately.

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

    Awesome article!

    I'm intrigued with the cap on the sequential reads that Intel has on the G2 drives as well. I always thought it was strange to see even on their first gen stuff.

    I'm assuming that this cap might be in place to somehow ensure the excellent performance they are giving with random read/writes. All until TRIM finally shows up and you'll have to write up another full on review (which I eagerly await!).

    I can't wait to see what 2010 brings to the table. What with the next version of SATA and TRIM just over the horizon, I could finally get the kind of performance out of my PC that I want!!
    Reply

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