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

251 Comments

View All Comments

  • jay401 - Wednesday, March 18, 2009 - link

    yeah, he wants "more expensive than" or "too expensive for". Reply
  • Spoelie - Wednesday, March 18, 2009 - link

    Second page as well:

    missing charts before and after this paragraph:

    "The chart above shows how much faster these affordable MLC SSDs were than the fastest 3.5” hard drive in sequential transfers. But now look at random write performance:"
    Reply
  • Spoelie - Wednesday, March 18, 2009 - link

    chart 1 on page 2 now shows sequential read but the paragraph is changed to mention random read ;)

    page 21: As far as I know, this is THE one of THE only reviews

    Some very surprising benchmark results for the ocz vertex, I thought the new firmware tanked sequential read speeds (to 80-90) based on the explanation beforehand, but not according to the actual graphs.
    Reply
  • Spoelie - Wednesday, March 18, 2009 - link

    third page, first table, first column: SSD and HDD entries are switched Reply
  • mikaela - Tuesday, March 16, 2010 - link

    yeah great info. also great resource Reply
  • Spoelie - Wednesday, March 18, 2009 - link

    page 19: I’d never reviewed it
    'd & -ed?
    Reply
  • HolyFire - Wednesday, March 18, 2009 - link

    "I'd never reviewed it" is correct. "I'd" here means "I had", it's Past Perfect tense. Reply
  • FishTankX - Wednesday, March 18, 2009 - link

    That should have bolded "too" Reply
  • FishTankX - Wednesday, March 18, 2009 - link

    Also, I think the velociraptor vs X-25 figures are swapped. 6 odd ms for the intel drive and 0.11ms for the velociraptor.. Reply
  • Natfly - Wednesday, March 18, 2009 - link

    Reply

Log in

Don't have an account? Sign up now