The Blind SSD

Modern OSes talk to hard drives using logical block addressing. While hard drives are rotational media, logical block addressing organizes sectors on a hard drive linearly. When you go to save a file, Windows simply issues a write command for your file at a specific logical block address, say LBA 15 for example.

Your OS knows what LBAs are available and which ones are occupied. When you delete a file, the LBAs that point to that file on your hard disk are listed as available. The data you’ve deleted hasn’t actually been removed and it doesn’t get wiped until those sectors on the drive are actually overwritten.

Believe it or not, SSDs actually work the same way.

The flash translation layer in a SSD controller maps LBAs to pages on the drive. The table below explains what happens to the data on the SSD depending on the action in the OS:

Action in the OS Reaction on a HDD Reaction on an SSD
File Create Write to a Sector Write to a Page
File Overwrite Write new data to the same Sector Write to a Different Page if possible, else Erase Block and Write to the Same Page
File Delete Nothing Nothing

 

When you delete a file in your OS, there is no reaction from either a hard drive or SSD. It isn’t until you overwrite the sector (on a hard drive) or page (on a SSD) that you actually lose the data. File recovery programs use this property to their advantage and that’s how they help you recover deleted files.

The key distinction between HDDs and SSDs however is what happens when you overwrite a file. While a HDD can simply write the new data to the same sector, a SSD will allocate a new (or previously used) page for the overwritten data. The page that contains the now invalid data will simply be marked as invalid and at some point it’ll get erased.

Strength in Numbers, What makes SSDs Fast Understanding the SSD Performance Degradation Problem
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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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