Strength in Numbers, What makes SSDs Fast

Given the way a single NAND-flash IC is organized one thing should come to mind: parallelism.

Fundamentally the flash that’s used in SSDs cut from the same cloth as the flash that’s used in USB drives. And if you’ve ever used a USB flash drive you know that those things aren’t all that fast. Peak performance to a single NAND-flash IC is going to be somewhere in the 5 - 40MB/s range. You get the faster transfer rates by reading/writing in parallel to multiple die in the same package.

The real performance comes from accessing multiple NAND ICs concurrently. If each device can give you 20MB/s of bandwidth and you’ve got 10 devices you can access at the same time, that’s 200MB/s of bandwidth. While hard drives like reads/writes to be at the same place on the drive, SSDs don’t mind; some are even architected to prefer that data be spread out all over the drive so it can hit as many flash devices as possible in tandem. Most drives these days have 4 - 10 channel controllers.

The Recap

I told you I’d mention this again because it’s hugely important, so here it is:

A single NAND flash die is subdivided into blocks. The typical case these days is that each block is 512KB in size. Each block is further subdivided into pages, with the typical page size these days being 4KB.

Now you can read and write to individual pages, so long as they are empty. However once a page has been written, it can’t be overwritten, it must be erased first before you can write to it again. And therein lies the problem, the smallest structure you can erase in a NAND flash device today is a block. Once more, you can read/write 4KB at a time, but you can only erase 512KB at a time.

It gets worse. Every time you erase a block, you reduce the lifespan of the flash. Standard MLC NAND flash can only be erased 10,000 times before it goes bad and stops storing data.

Based on what I’ve just told you there are two things you don’t want to do when writing to flash: 1) you don’t want to overwrite data, and 2) you don’t want to erase data. If flash were used as a replacement for DVD-Rs then we wouldn’t have a problem, but it’s being used as a replacement for conventional HDDs. Who thought that would be a good idea?

It turns out that the benefits are more than worth the inconvenience of dealing with these pesky rules; so we work around them.

Most people don’t fill up their drives, so SSD controller makers get around the problem by writing to every page on the drive before ever erasing a single block.

If you go about using all available pages to write to and never erasing anything from the drive, you’ll eventually run out of available pages. I’m sure there’s a fossil fuel analogy somewhere in there. While your drive won’t technically be full (you may have been diligently deleting files along the way and only using a fraction of your drive’s capacity), eventually every single block on your drive will be full of both valid and invalid pages.

In other words, even if you’re using only 60% of your drive, chances are that 100% of your drive will get written to simply by day to day creation/deletion of files.

The Anatomy of an SSD The Blind SSD
POST A COMMENT

358 Comments

View All Comments

  • KadensDad - Tuesday, October 27, 2009 - link

    How do these drives fail? I have heard that they will just suddenly die, no more writes or reads possible. What I would like to know is what happens when it dies? Do you lose all data? Just can't write anymore? How does the OS respond? Any early warnings? What about e.g. CRC? How does possibility of data corruption compare to traditional SSD? What about RAID? Since the drives are electrical, not mechanical, this reduces the number of failure vectors and environmental concerns (e.g., ambient temperature over lifetime of the drive). Won't SSDs therefore fail closer together in time in a RAID configuration? This reduces the window of opportunity for fixing an array and also decreases the applicability of RAID, however marginal.
    Reply
  • Dewend - Friday, March 04, 2016 - link

    My partner and i also were seated for lunch, whenever i mentioned to her that I read a script each and every morning newspaper, therefore i chosen to do a little research. Thankfully, I stumbled upon this website, which helped me discover why people consider even thinking about this. http://followersteam.com Reply
  • Askipod - Thursday, November 17, 2016 - link

    This is cool! Your website is great Hey Your information is astounding!! I will recommend it to my brother and anybody that could be attracted to this topic. Reply
  • Askipod - Thursday, November 17, 2016 - link

    WOW. I am very glad to join this comunity and enjoy your article. Your article shared me so interesting information here, looking forward to your update. http://www.nasze.media.pl/pisanie-tekstow-jako-spo... Reply
  • echtgeld - Monday, January 16, 2017 - link

    http://echtgeld-casino.net/ - beste online casinos site Reply
  • SecXploded - Saturday, February 25, 2017 - link

    Exactly. you’re very kind of us about comment - <a href="http://www.nohumanverification.com">www.no... Reply
  • SecXploded - Saturday, February 25, 2017 - link

    Exactly. you’re very kind of us about comment - www.nohumanverification.com Reply
  • SecXploded - Saturday, February 25, 2017 - link

    Exactly. you’re very kind of us about comment - http://www.nohumanverification.com Reply
  • vitaliy915osipov - Wednesday, July 19, 2017 - link

    this is working http://www.cetgames.com/ Reply
  • ultrafine99 - Wednesday, April 12, 2017 - link

    My profile pic looks like im twelve but that was taken 6 years ago.
    http://www.givemegiftcodes.com/
    Reply

Log in

Don't have an account? Sign up now