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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  • Hrel - Thursday, April 09, 2009 - link

    Instead of making me dinner can you send me that test system instead??? Please!!! Reply
  • Hrel - Thursday, April 09, 2009 - link

    I was wondering what controller the OCZ solid Series is based on??? Will I experience hiccups with that drive or not? Is the point of my question. Reply
  • sfisher64 - Wednesday, April 08, 2009 - link

    I just purchased a Dell Latitude E6400 with a 64GB Ultra Performance Solid State Drive. Does anyone know what type of drive this is, and where it fits in the spectrum described in this article? Reply
  • Baffo - Saturday, April 11, 2009 - link

    The Dells use the Samsung drives (you should see this on the bottom if you pull it out). However, as much as I wish this was one of the newer controllers (I have a few of these at work as well), the testing cycles demanded by Dell probably mean these are the older controllers. Reply
  • marraco - Tuesday, April 07, 2009 - link

    This article is popular :) Reply
  • BLHealthy4life - Monday, April 06, 2009 - link

    Intel 9.1.1.1010 (Intel) Where are these drivers? I can only find version 1007 and not 1010....

    Thanks
    Reply
  • BLHealthy4life - Sunday, April 12, 2009 - link

    found it....

    Intel obviously keeps the X58 chipset drivers current for their own boards, just not other mfgs boards....

    They installed fine on my R2E..

    BL
    Reply
  • irondukes - Friday, April 03, 2009 - link

    Hi-- Do SLCs suffer from performance degradation, or are the controllers pretty agressive at erasing the data since they have far longer read-write cycles? Please help! Deciding between an X25E and X25M Reply
  • mdavies - Friday, April 03, 2009 - link

    I'm reading this about a day late - got my Patriot PE256GS25SSDR 2.5" 256GB yesterday since I'm bad about destroying hard drives. this drive, in a word, was excruciating. I'll be replacing it with one of your recommended drives today.

    Thanks
    Reply
  • sotoa - Friday, April 03, 2009 - link

    Long time reader, first time post.
    I really liked the background story and appreciate how Anand delves deep into the the SSD's (as well as other products in other articles).

    Thanks for looking out for the little guy!
    Keep up the great work!
    Reply

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