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

313 Comments

View All Comments

  • JarredWalton - Wednesday, March 18, 2009 - link

    Yes, *but* SSD could also be read as "Solid State Drive" instead of "ess ess dee", in which case you would say "a SSD". I tend to read it as "ess ess dee", but Anand thinks of those letters as "Solid State Drive".

    Potato, potato, tomato, tomato... let's call the whole thing off!
    Reply
  • Azsen - Thursday, March 19, 2009 - link

    When reading acronyms you're supposed to think of them as the letters, i.e. when you see RAM, you think "ram" straight off not Random Access Memory. When you see "IBM" you think "eye bee emm" not International Business Machines etc etc. It would take ages to read an article if you had to stop and think out all the full wording of acronyms as you're reading them.

    I'm going with the correction of "Anatomy of an SSD". Correct English fullstop.
    Reply
  • JarredWalton - Thursday, March 19, 2009 - link

    By your comment, you suggest two different things, and that's really okay. That was my point: when you see "RAM" you probably thing "ram" as in the animal... not "Are A Em". You say "a RAM stick" not "an RAM stick". I'd guess most people think of SATA as "Ess A Tee A", but if you talk to most computer techs that are in the know, it's "say-te" so you would say "a SATA drive".

    And you know, I'm sure plenty of people will agree with the correct way of saying SATA, and that's perfectly okay. English really is a very flexible thing - particularly in the tech world - and rarely is there an "always right" way of saying things. If Anand wants to say "a SSD" and others want to say "an SSD", I'm not going to try to declare one group or the other correct. They both are, depending on your viewpoint.

    "I believe the world is neither black nor white, but only shades of gray."
    Reply
  • Pythias - Friday, March 20, 2009 - link

    Can't have gray without black and white. Reply
  • 7Enigma - Wednesday, March 18, 2009 - link

    HAHAHA. What a tool. I love it when people critique grammar.....and get it wrong. Reply
  • VaultDweller - Wednesday, March 18, 2009 - link

    I love it when people critique someone's critique of grammar... and get it wrong.

    It's an SSD, not a SSD.
    Reply
  • gwolfman - Wednesday, March 18, 2009 - link

    lolz Reply
  • sidex - Wednesday, March 18, 2009 - link

    I would like to know the firmware version of Vertex used in your review. To me sounds the old 0112 Reply
  • kensiko - Wednesday, March 18, 2009 - link

    Yes that is important to know.

    I'm sure this is not done with the latest firmware available which is 1199. This version got better performance.

    Firmware 1275 is coming also.

    Anand, will you update your benchmarks with the latest firmware?

    If not, then the benchmarks are obsolete.
    Reply
  • Anand Lal Shimpi - Wednesday, March 18, 2009 - link

    I tested with the shipping firmware for this article (0122). I've been playing around with 1199 in the lab and will most likely have an update in a couple of weeks once I've done a thorough evaluation of it. By then I should also have the final version of the new Samsung drive and maybe even some other interesting things.

    For now, I've got to get to work on the new Mac Pro and the updated Ion article :) I need a small break from SSDs por favor :)

    Take care,
    Anand
    Reply

Log in

Don't have an account? Sign up now