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.

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  • strikeback03 - Wednesday, March 25, 2009 - link

    I was wondering the same thing. I'd guess it would be a lot of writing/erasing, so an SSD might not be the best from a longevity standpoint, but if your system is hitting the scratch disk often then the speed might make it worthwhile. Reply
  • mikepers - Tuesday, March 24, 2009 - link

    Anand,

    I wanted to compliment you on what I think was an excellent article. This is the type of thing I really have always liked from Anandtech. The detailed background, the technical reasons for the issues and then a thorough review of the current state of things.

    I just finished upgrading my desktop. The only remaining item I wanted to replace was the hard disk. I had been thinking about getting a Velociraptor but instead I just ordered a 60GB Vertex from Newegg.

    Thanks again for all the work.

    Mike P.
    Reply
  • ameatypie - Monday, March 23, 2009 - link

    That sure was a lot to take in! Fantastic article though, it has really opened my eyes to the possibilities that Solid State Drives provide. Probably wont be buying one in the immediate future given the so-called depression and such things, but i will certainly keep up with SSD progress.
    Thanks again for your fantastic articles - im sure im not the only one who really appreciates them :)
    Reply
  • coopchennick - Monday, March 23, 2009 - link

    Hey Anand, I just finished reading through this whole article and I'm very impressed with the thoroughness and how informative it was.

    You just acquired a new regular reader.
    Reply
  • zdzichu - Sunday, March 22, 2009 - link

    Very nice and thorough article. I only lack more current status of TRIM command support in current operating systems. For example, Linux supports it since last year:

    http://kernelnewbies.org/Linux_2_6_28#head-a1a9591...">http://kernelnewbies.org/Linux_2_6_28#h...a9591f48...
    Reply
  • Sinned - Sunday, March 22, 2009 - link

    Outstanding article that really helped me understand SSD drives. I wonder how much of an impact the new SATA III standard will have on SSD drives? I believe we are still at the beginning stage for SSD drives and your article shows that much more work needs to be done. My respect for OCZ and how they responded in a positive and productive way should be a model for the rest of the SSD makers. Thank you again for such a concise article.
    Respectfully,
    Sinned
    Reply
  • 529th - Sunday, March 22, 2009 - link

    The first thing I thought of was Democracy. Don't know why. Maybe it was because a company listened to our common goal of performance. Thank you OCZ for listening, I'm sure it will pay off!!! Reply
  • araczynski - Saturday, March 21, 2009 - link

    very nice read. the 4/512 issue seems a rather stupid design decision, or perhaps more likely a stupid problem to find this 4/512 solution as 'acceptable'.

    although a great marketing choice, built in automatic life expectancy reduction.

    sounds like the manufacturers want the hard drives to become a disposable medium like styrofoam cups.

    perhaps when they narrow the disparity down to 4/16, i might consider buying an ssd. that, or when they beat the 'old school' physical platters in price.

    until then, get back to the drawing board and stop crapping out these half arsed 'should be good enough' solutions.
    Reply
  • IntelUser2000 - Sunday, March 22, 2009 - link

    araczynski: The 4/512 isn't done by accident. It's done to lower prices. The flash technology used in SSDs are meant to replace platter HDDs in the future. There's no way of doing that without cost reductions like these. Even with that the SSDs still cost several times more per storage space. Reply
  • araczynski - Tuesday, March 24, 2009 - link

    i understand that, but i don't remember original hard drives being released and being slower than the floppy drives they were replacing.

    this is part of the 'release beta' products mentality and make the consumer pay for further development.

    the 5.25" floppy was better than the huge floppy in all respects when it was released. the 3.5" floppy was better than the 5.25" floppy when it was released. the usb flash drives were better than the 3.5" floppies when they were released.

    i just hate the way this is being played out at the consumer's expense.
    Reply

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