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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  • Iger - Wednesday, March 25, 2009 - link

    This is the best review I've read in a very long time.
    Thank you very much!
    Reply
  • BailoutBenny - Tuesday, March 24, 2009 - link

    Great in depth article on flash based SSDs. I'm waiting for PRAM though. Reply
  • orclordrh - Tuesday, March 24, 2009 - link

    Very illuminating article, very well written and researched. It made me glad that I didn't pull the trigger on an SSD for my I7 machine and regret not buying OCZ memory! I'm interested in adding an SSD as the scratch disk for Photoshop CS4 to use. I don't really launch applications very often, say once a week on the weekly reboot and keep 6-8 apps open at all times. I have 12GB of memory for that. The benchmarks were very interesting, but what sort of activity does Photoshop scratch usage create? Large files or random writes? What type of SSD would be most cost effective here?
    An SSD does sound better than a SSD!
    Reply
  • semo - Wednesday, March 25, 2009 - link

    wait for ddr3 to enter the mainstream and buy loads of memory.

    use a ramdisk for your adobe scratch area. much faster than ssd and no wear to worry about (not that you would worry that much with modern ssds anyway).

    http://www.ghacks.net/2007/12/14/use-a-ramdisk-to-...">http://www.ghacks.net/2007/12/14/use-a-ramdisk-to-...

    there is also a paid for and more feature rich ramdisk out there. can't remember the name
    Reply
  • strikeback03 - Wednesday, March 25, 2009 - link

    I'll have to check when I get home, but I believe the recommended size for the scratch disk is upwards of 10GB. So would need a motherboard that supports a LOT of RAM to give enough to main memory plus a scratch disk. Reply
  • 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

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