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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  • Gasaraki88 - Friday, March 20, 2009 - link

    This truly was a GREAT article. I enjoyed reading it and was very informative. Thank you so much. That's why Anandtech is the best site out there. Reply
  • davidlants - Friday, March 20, 2009 - link

    This is one of the best tech articles I have ever read, I created an account just to post this comment. I've been a fan of Anandtech for years and articles like this (and the RV700 article from a while back) show the truly unique perspective and access that Anand has that simply no other tech site can match. GREAT WORK!!! Reply
  • Zak - Friday, March 20, 2009 - link

    I just got the Apex. I'd probably cough up more dough for the Vertex after reading this. However, I've run it for two days as my system disk in MacPro and haven't noticed any issues, it's really fast. But I guess I'll get Vertex for my Windows 7 build.

    Z.
    Reply
  • Nemokrad - Friday, March 20, 2009 - link

    What I find intriguing about this article is that these smaller manufacturers do not do real world internal testing for these things. They should not need 3rd parties like you to figure this shit out for them. Maybe now OCZ will learn what they need to do for the future. Reply
  • JonasR - Friday, March 20, 2009 - link


    Thanks for an excellent article. I have one question does anyone know which controller is beeing used in the new Patriot 256GB V.3 SSD?
    Reply
  • tgwgordon - Friday, March 20, 2009 - link

    Anyone know if the Vertex Anand used had 32M or 64M cache? Reply
  • Dennis Travis - Friday, March 20, 2009 - link

    Excellent and informative article as always Anand. Thanks so much for posting the truth!! Reply
  • IsLNdbOi - Friday, March 20, 2009 - link

    Can't remember what page it was, but you showed some charts on the performance of SSDs at their lowest possible performance levels.

    At their lowest possible performance levels are they still faster than the 300GB Raptor?
    Reply
  • Edgemeal - Friday, March 20, 2009 - link

    It's too bad Windows and applications don't let you select where all the data that needs to be updated and saved to is stored. If that was an option a SSD could be used to only load data (EXE files and support files) and a HDD could be used to store files that are updated frequently, like a web browser for example, their constantly caching files, from the sound of this article that would kill the performance of a SSD in no time.

    Great article, I'll stick to HDDs for now.
    Reply
  • Luddite - Friday, March 20, 2009 - link

    So even with the TRIM command, when working with large files, say, in photoshop and saving multiple layers, the performance will stil drop off? Reply

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