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
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  • Gasaraki88 - Friday, March 20, 2009 - link

    RAID is a universal standard so if you take two RAID0 drives out and move them to another computer with a RAID controller, it SHOULD just work if the original RAID was doing it correctly. Reply
  • coil222 - Wednesday, March 18, 2009 - link

    Yes I run a pair of MTRON 7500s in a raid 0 stripe for my OS and Gaming (wow). I don't recall numbers off the top of my head but tests were better on the raid 0 than a single drive configuration.

    Watch this:
    http://www.youtube.com/watch?v=96dWOEa4Djs&fea...">http://www.youtube.com/watch?v=96dWOEa4Djs&fea...
    Reply
  • sawyeriii - Wednesday, March 18, 2009 - link

    I just wanted to state how much I loved the combination of technical and real world information in this article.

    What is the possibility of having different page sizes built into a drive? I.e. you could have a drive with many 1k page packages on one die, 2k on another, and most others 4k. Could that theoretically help? Could the controllers work with that (or would you need to combine multiple 1k's into a 4k transfer size)?

    PS One note on page 3, the VelociRaptor and Intel in the first chart (responce time) are switched, however the text is correct.
    Reply
  • StormyParis - Wednesday, March 18, 2009 - link

    the ugly truth is that an SSD won't let you do anything that you couldn't do without it, and due to its cost and small capacity, it's not a replacement drive, it's an extra drive: not less power consumption but more, not less noise but just the same. You just gain a bit of time when booting up and lauching apps... which I do about 1/week and 1/day, respectively. Assuming your system has enough RAM (and if it doesn't, buy RAM before buying an SSD !), you won't feel much difference once the apps are launched.

    For the same cost, I'd rather buy a bigger screen.

    It's urgent to wait for prices to come down. But I'm all for lots of people buying them now and help get the price down for us wiser buyers.
    Reply
  • Rasterman - Thursday, March 19, 2009 - link

    I've already decided my next system in a few months will have one, after you go through 5 hard drive failures (over several years) lets see how much your willing to pay to not have to put up with it anymore. If you use your PC for anything useful (work) then an SSD is a no brainer even at $1000/64GB IMO if the data security is there, speed is secondary for me.

    When you already have the best screen, video card, memory, why not have the best drive? And your argument is pretty dumb, almost any upgrade won't let you do anything that you couldn't do without it, not just SSDs.
    Reply
  • Calin - Wednesday, March 18, 2009 - link

    You get lower power due to the lower power use of the SSD and the fact that the other drive is not stressed with difficult access patterns (small random reads/writes). Remember that idle power of a SSD drive is very low Reply
  • 7Enigma - Wednesday, March 18, 2009 - link

    No, his comment was accurate for most users. Due to the small capacities and high cost these will be used as boot drives primarily with maybe a single heavily used program (say the current game or program you are playing/using), the rest will be on an additional drive. So while the power consumption of the SSD would be less than the old drive, the aggregate power usage of both (even when the larger storage drive is primarily at idle) will be higher than the single HD.

    And I believe you meant to say traditional HD for idle power?
    Reply
  • strikeback03 - Wednesday, March 18, 2009 - link

    If all you were going to throw on the drive is the OS and a game, a 32GB drive should be plenty. The reason the 80GB and up range is important is so general consumers can load all their programs on it.

    But yes, in consumer usage other than a laptop, some people who were previously using one drive for both boot and storage would likely need a mechanical HDD is addition to the SSD. OTOH, those who were using a Velociraptor (or RAID array) for boot and another drive for storage will see their power consumption decrease.
    Reply
  • sawyeriii - Wednesday, March 18, 2009 - link

    Have you used a SSD? (If so which)

    I would state that it is not a luxary product, it is a premium product. The price difference you pay WILL translate to faster performance (if you choose correctly). More RAM only helps upto a point.

    Remember performance is based on a system of parts...
    CPU
    RAM
    NORTHBRIDGE
    GPU
    SOUTHBRIDGE
    I/O INTERFACE
    HDD/SDD

    Microsoft's Windows Experience Index has specific flaws, but the concept is sound... The system can only go a fast as the slowest component in the system (relative to the amount of time used by that component).
    Reply
  • Testtest - Wednesday, March 18, 2009 - link

    ... there's also Supertalent's Ultradrive ME (MLC) and LE (SLC) and Photofast's G-Monster v3

    At least the Supertalent drives are quite a bit cheaper with the same drive layout/controller than the Vertex drives and only differ in the firmware (which isn't bad either).

    It's however possible at least with the Ultradrive ME currently to provoke a kinda timeout error after they've been fully filled once and then still beeing written on. I don't own a Vertex so I can't test that there but if it was a controller issue, it should pop up there sooner or later as well (if you take a look in their suppport forum some error reports seem very similar).

    Intels have their 80% bug, Indilinx drives have their issues too it seems - let's hope that firmware can cure it!

    Great article btw!
    Reply

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