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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  • Slash3 - Wednesday, March 18, 2009 - link

    Page 29: "Not all applications will launch faster than a VelociRaptor on a SSD, but let's not forget that the VelociRaptor is the world's fastest hard drive."

    Really? What about the nice and speedy enterprise-level 15k SAS/SCSI drives everyone neglects to acknowledge? :)
    Reply
  • George Powell - Wednesday, March 18, 2009 - link

    I believe it refers to consumer drives. While SAS drives are beginning to be a possibility on the desktop with newer motherboards supporting them natively, the drives themselves are too expensive and too noisy for most consumers to actually want them.


    Reply
  • FishTankX - Wednesday, March 18, 2009 - link

    Good info. However, I noticed one mistake.

    Second page
    Samsung had a MLC controller at the time but it was too expensive than what SuperTalent was shooting for.
    Reply
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  • matrixireland - Thursday, December 24, 2009 - link

    hi would like to know what you pros think of the;
    Golden Leopard ASAX-ZIF1.8-SSD? what would you add to it?
    And how would you rate it against other ssd?

    Specifications:

    product description

    ASAX-ZIF1.8-SSD is a high-performance design solid state drive based on the high-end micro-control IC with flash memory storage medium integrated advantaged of high speed,convenient ,aseismatic,energy-saving etc.


    specification

    Model

    Size

    Interface

    Material
    ASAX-ZIF1.8-SSD

    1.8inch 70×54×6mm

    ZIF
    Aluminum-magnesium alloy appearance ,drawbench and colorful oxidation surface,elegant temperament


    performance
    read speed:80- 96Mbytes/second write speed:50- 60Mbytes/second
    support ATA-7 V3 PIO/multi word/ultra DMA MODES
    Low power TFBGA,4 channel of flash controller,masked ROM and data SRAM
    SAMSUNG flash keeps the data faster on reliability and endurance
    Dynamic and static wear-leveling prolong NAND FLASH and SSD for longer life
    8/16 bit BCH ECC data error correction ability effectively guarantee the data read security.

    Design consideration

    Capacity

    16G/32G/64G/128G/256G
    Average access time

    <0.25MS
    operating temperature

    0-85°
    power consumption

    DC Input Voltage(3.3V or 5 V ± 10%)Read and write:135mA/194Ma wait:70mA
    shock

    1500G


    Application
    the Laptop, pc, server,workstation,portable media player,digital collection apparatus and any computer equipment which need consecutive read and write speed and high reliability storage.
    Reply
  • jay401 - Wednesday, March 18, 2009 - link

    yeah, he wants "more expensive than" or "too expensive for". Reply
  • Spoelie - Wednesday, March 18, 2009 - link

    Second page as well:

    missing charts before and after this paragraph:

    "The chart above shows how much faster these affordable MLC SSDs were than the fastest 3.5” hard drive in sequential transfers. But now look at random write performance:"
    Reply
  • Spoelie - Wednesday, March 18, 2009 - link

    chart 1 on page 2 now shows sequential read but the paragraph is changed to mention random read ;)

    page 21: As far as I know, this is THE one of THE only reviews

    Some very surprising benchmark results for the ocz vertex, I thought the new firmware tanked sequential read speeds (to 80-90) based on the explanation beforehand, but not according to the actual graphs.
    Reply
  • Spoelie - Wednesday, March 18, 2009 - link

    third page, first table, first column: SSD and HDD entries are switched Reply
  • mikaela - Tuesday, March 16, 2010 - link

    yeah great info. also great resource Reply

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