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

235 Comments

View All Comments

  • Natfly - Wednesday, March 18, 2009 - link

    Reply
  • DangerMouse4269 - Tuesday, April 13, 2010 - link

    Nicely written. Even a very out of practice Comp Eng understood that. Reply
  • geekforhire - Monday, June 14, 2010 - link

    I have just replaced the hard drive in this 3 year old Dell Inspiron 9400 notebook computer with a new and very quick OCZ SSD, manually configured the partition with a 1024 offset, freshly installed the OS, freshly downloaded all of the latest and greatest drivers from Dell, and applied all currently available OS updates from Msft.

    The problem is that when the machine resumes from Standby, it will /reliably/ (4 out of 4 attempts) produce a BSOD 0xF4 after the power button is pressed to resume the machine from standby.

    Here's the sequence to recreate the problem:

    0) Machine is booted normally into Windows, and log in to an account which has administrative privs.
    1) Click on Start -> Shut Down -> Standby.
    2) See display turn black, disk I/O light flashes then stops, then the power indicator light begins to flash on and off slowly.
    3) Wait until the power light has made 2 slow flashes.
    4) Press the power button.
    5) See the Dell Bios splash screen, then disappear
    6) Boom: See the BSOD 0xF4

    The values reported after the STOP are:
    (0x00000003, 0x865b3020, 0x865b3194, 0x805d2954)

    Note that I've been in contact with OCZ before about this SSD+computer, because the previous BSOD that was produced was 0x77. Their recommendation was to create the partition with an offset with a 64 interval, and to reflash the SSD with their modern firmware. This was done, the OS was reinstalled as described, and now I'm getting a different BSOD code. Another mention was a question whether the notebook computer uses a SATA2 controller (definitely compatible) or SATA1 (which may have troubles).

    I've run Spinrite on the SSD, and there are lots of ECC errors being reported. I've been in contact with Spinrite, and they chalk this up to the SSD being chatty (which they like), but since SSD's are new and magnetic disks are common, they want to stay focussed on magnetic disks.

    When the machine boots back up, the OS reports that a serious error has occurred, and asks that a problem report be submitted, which I do. Then an attractive but somewhat generic page is displayed with common causes (Aging or failing hard disks, large file transfers from secondary media to local hd, loss of power to a hard drive, hard disk intensive processes (eg: antivirus scanners), recently installed hardware that might have compatibility and performance problems)

    Has anyone else encountered this kind of problem, and do you have any suggestions?
    Reply
  • angavar - Thursday, September 09, 2010 - link

    As a medical student I can appreciate a well researched and analytical article when I see it. This is by far the best computer hardware review I have ever read! Thank-you for your time and effort in producing what is clearly a thoroughly researched and detailed analysis. Reply
  • mac021 - Wednesday, October 17, 2012 - link

    Thank you for the lesson and helping me understand SSD drives. May I just ask for your advice...

    For everyday use designing and generating prototypes for websites and running typical office s/w like word and excel for long documentations while listening to music or just having some video play in the background, then the occasional gaming of, say Star Craft 2 and Dead Space 3, and lets assume I do this on a 5 hours a day average for 365 days in a year, how long before I need to replace an OCZ Vertex/Summit SSD? And does format/reinstall help in prolonging the life of an SSD just as it does for my old hard drives (from a computer that's 6 years old and counting)? Or there's no stopping the SSD's death after reaching 10,000 times of being erased and rewritten on? I'm not one who keeps upgrading or buying new computer systems for every new thing that comes out, i'm more of a keeper and maintainer for as long as the system servers my needs... but when I make a purchase, I make sure it will be enough to last me another 6-12 years IF possible! Which is why I'm still considering SATA for my next purchase late this year or early next year (and I'm only buying a new PC just because I made a mistake buying a foxconn motherboard that can't support anything higher than XP, not even Vista... weird, anyway I found that out too late).

    Also, would you know of a motherboard that supports SSD, Windows 8, Nvidea, third gen i5/i7, and up to 64GB ram?

    Thanks so much!
    Reply

Log in

Don't have an account? Sign up now