A Quick Flash Refresher

DRAM is very fast. Writes happen in nanoseconds as do CPU clock cycles, those two get along very well. The problem with DRAM is that it's volatile storage; if the charge stored in each DRAM cell isn't refreshed, it's lost. Pull the plug and whatever you stored in DRAM will eventually disappear (and unlike most other changes, eventually happens in fractions of a second).

Magnetic storage, on the other hand, is not very fast. It's faster than writing trillions of numbers down on paper, but compared to DRAM it plain sucks. For starters, magnetic disk storage is mechanical - things have to physically move to read and write. Now it's impressive how fast these things can move and how accurate and relatively reliable they are given their complexity, but to a CPU, they are slow.

The fastest consumer hard drives take 7 milliseconds to read data off of a platter. The fastest consumer CPUs can do something with that data in one hundred thousandth that time.

The only reason we put up with mechanical storage (HDDs) is because they are cheap, store tons of data and are non-volatile: the data is still there even when you turn em off.

NAND flash gives us the best of both worlds. They are effectively non-volatile (flash cells can lose their charge but after about a decade) and relatively fast (data accesses take microseconds, not milliseconds). Through electron tunneling a charge is inserted into an N-channel MOSFET. Once the charge is in there, it's there for good - no refreshing necessary.


N-Channel MOSFET. One per bit in a NAND flash chip.

One MOSFET is good for one bit. Group billions of these MOSFETs together, in silicon, and you've got a multi-gigabyte NAND flash chip.

The MOSFETs are organized into lines, and the lines into groups called pages. These days a page is usually 4KB in size. NAND flash can't be written to one bit at a time, it's written at the page level - so 4KB at a time. Once you write the data though, it's there for good. Erasing is a bit more complicated.

To coax the charge out of the MOSFETs requires a bit more effort and the way NAND flash works is that you can't discharge a single MOSFET, you have to erase in larger groups called blocks. NAND blocks are commonly 128 pages, that means if you want to re-write a page in flash you have to first erase it and all 127 adjacent pages first. And allow me to repeat myself: if you want to overwrite 4KB of data from a full block, you need to erase and re-write 512KB of data.

To make matters worse, every time you write to a flash page you reduce its lifespan. The JEDEC spec for MLC (multi-level cell) flash is 10,000 writes before the flash can start to fail.

Dealing with all of these issues requires that controllers get very crafty with how they manage writes. A good controller must split writes up among as many flash channels as possible, while avoiding writing to the same pages over and over again. It must also deal with the fact that some data is going to get frequently updated while others will remain stagnant for days, weeks, months or even years. It has to detect all of this and organize the drive in real time without knowing anything about how it is you're using your computer.

It's a tough job.

But not impossible.

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  • Jedi2155 - Monday, August 31, 2009 - link

    Anandtech has always been known for its in-depth analysis, you're just looking for a simple review list. I much prefer these detailed articles than just hearing the list of performance and simple recommendations that most people can write if provided the proper hardware.

    I love how Anand always writes excellent, very well detailed articles that are still SIMPLE to understand. A number of other sites may offer some similar levels of detailed but are sometimes a bit too difficult to comprehend without a background in the same field.
    Reply
  • KommisMar - Sunday, April 4, 2010 - link

    Anand,

    I read your long series of articles on SSDs today, and just wanted to say thanks for writing the most informative and interesting series of tech articles I've read in years. I've been avoiding SSDs because my first experience with one was horrible. The sustained transfer rates were no better than a traditional hard drive, and the system halting for several seconds on each random write operation was too much for me to stand.

    I was so sick of the SSD coverage that I was reading on other websites because none of them seemed to answer my biggest question, which was "Which SSD won't bring my system to a screeching halt every time it needs to write a little data?"

    Thanks for answering that question and explaining what to look for and what to avoid. It sounds like it's a good time for me to give SSDs another shot.
    Reply
  • jamesy - Thursday, April 22, 2010 - link

    That about sums it up: disappointment. Although this was a top-caliber SSD article, like i have come to love and expect out of anand, this article didn't make my buying decision any easier as all. In fact, it might have made it more complicated.

    I understand Intel, Indillinx, and Sandforce are good, but there are so many drives out there, and most suck. This article was amazing by most standards but the headline should be changed: removing the "Choosing the Best SSD."

    Maybe "Choosing the right controller before sorting through a hundred drives" would be an appropriate replacement.

    Do i still go with the intel 160 X-25m G2?
    Do I get the addon Sata 6g card and get the C300?
    Do i save the money, and get an indillinx drive? Is the extra money worth the Intel/C300 drive?

    These are the main questions enthusiasts have, and while this article contained a great overview of the market in Q3 2009, SSD Tech has progressed dramatically. Only now, i think, are we getting to the point that we could publish a buying guide and have it last a few months.

    I trust Anandtech, i just wish they would flat-out make a buying guide, assign points in different categories (points for sequential read/write, points for random read/write, points for real-life performance or perceived performance, points for reliability, and points for price.). Take all of these points, add em up, and make a table pls.

    A few graphs can help, but the 200 included in each article is overwhelming, and does nothing to de-complicate or make me confindent in my purchase.

    It's great to know how drives score, how they perform. But it's even important to know that you bought the right drive.
    Reply
  • mudslinger - Monday, June 28, 2010 - link

    This article is dated 8/30/2009!!!!
    It’s ancient history
    Since then newer, faster SSD’s have been introduced to the market.
    And their firmware have all been updated to address known past issues.
    This article is completely irrelevant and should be taken down or updated.
    I’m constantly amazed at how old trash info is left lingering about the web for search engines like Google to find. Just because Google lists an article doesn’t make it legit.
    Reply
  • cklein - Monday, July 12, 2010 - link

    Actually I am trying to find a reason to use SSD.
    1. Server Environment
    No matter it's a webserver or a SQL server, I don't see a way we can use SSD. My SERVER comes with plenty of RAM 32G or 64G. The OS/start a little bit slow, but it's OK, since it never stop after it's started. And everything is loaded into RAM, no page file usage is needed. So, really why do we need SSD here to boost the OS start time or application start time?
    For SQL server database, that's even worse. Let's say I have a 10GB SQL server database, and it grows to 50GB after a year. Can you image how many random writes, updates between the process? I am not quite sure this will wear off the SSD really quick.

    2. For desktop / laptop, I can probably say, install the OS and applications on SSD, and leave everything on other drives? And even create page file on other drives? As I feel SSD is only good for readonly access. For frequent write, it may wear off pretty quick? I am doing development, I am not even sure I should save source code on SSD, as it compiles, builds, I am sure it writes a lot to SSD.

    So over all, I don't see it fits in Server environment, but for desktop/laptop, maybe? even so, it's limited?

    someone correct me if I am wrong?
    Reply
  • TCQU - Thursday, July 29, 2010 - link

    Hi people

    I'm up for getting a new Macbook pro with ssd.
    BUT i heard somthing about, that the 128gb ssd, for apples machines, was made by samsung. I was ready to buy it, but now that i've heard that first of all "apples" ssd's is much slower that they others on the marked. Then i read this. So now i'm really confused.
    What shoud i do?
    buy apples macbook pro with 128gb ssd
    or should i buy it without and replace it with an other ssd? thoughts? plzz help me out
    thanks
    Thomas
    Reply
  • TCQU - Thursday, July 29, 2010 - link

    Hi people

    I'm up for getting a new Macbook pro with ssd.
    BUT i heard somthing about, that the 128gb ssd, for apples machines, was made by samsung. I was ready to buy it, but now that i've heard that first of all "apples" ssd's is much slower that they others on the marked. Then i read this. So now i'm really confused.
    What shoud i do?
    buy apples macbook pro with 128gb ssd
    or should i buy it without and replace it with an other ssd? thoughts? plzz help me out
    thanks
    Thomas
    Reply
  • marraco - Friday, August 13, 2010 - link

    Why Sandforce controllers are ignored?

    I’m extremely disappointed with the compiler benchmark. Please test .NET (With lot of classes source files and dependencies). It seems like nothing speeds up compilation. No CPU, no memory, no SSD. It makes nonsense.
    Reply
  • sylvm - Thursday, October 7, 2010 - link

    I found this article of very good quality.

    I was looking for a similar article about express card SSDs using PCIe port, but found nothing about their performance for rewrite.
    The best I found is this review http://www.pro-clockers.com/storage/192-wintec-fil... saying nothing about it.

    Expresscard SSDs would allow good speed improvement/price compromise : buying a relatively small and cheap one for OS and softwares, while keeping the HDD for data.

    Has anyone some info about it ?

    Best regards,

    Sylvain
    Reply
  • paulgj - Saturday, October 9, 2010 - link

    Well I was curious about the flash in my Agility 60GB so I opened it up and noted a different Intel part number - mine consisted of 8 x 29F64G08CAMDB chips whereas the pic above shows the 29F64G08FAMCI. I wonder what the difference is?

    -Paul
    Reply

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