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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  • albor - Friday, June 18, 2010 - link

    Hi,
    try RamDisk Plus 11 from SuperSpeed.
    (http://www.superspeed.com/desktop/ramdisk.php)
    I use it on Xp pro 32 bit with 30 GB OCZ Vertex and 8 GB RAM. All above 3.2 GB is configured for swap and temp. Works perfectly and no visible SSD performance degradation after about 10 months.
    Greetings.
    Reply
  • jmr3000 - Thursday, August 23, 2012 - link

    would explain to me how did you install it?

    the ssd as a second drive or did u install all the program on the ssd and use the hhd as a second?

    thanks in advance!

    jm
    Reply
  • marraco - Friday, August 13, 2010 - link

    SWAP file is one of the most important speed bottleneck on windows.

    it writes frequently to disk, so consumes the read write cicles of the disk, reducing his useful life.

    But you are not buying space storage when you buy SSD. You are buying speed, so it makes nosense to buy an expensive SSD, and then remove from it all the activities that need the speed and are bottlenecks.

    you buy a SSD to do the fastest SWAP. keep it on SSD.

    Also, drive indexing permanently does a lot of reads, but it does not matter if the disk is fast. Drive indexing is like a little local google. If you disable it, and then search for all the files with a given text on it, searching the entire disk takes longer than just read an indexing.

    Those activities consume the useful life of the disk, but at the time the disk gonna need replacement, (5 years, maybe), this disk gonna need replacement anyways, and new SSD gonna be dirty cheap, so it makes no sense to disable swap, temp files, and indexing.

    On other side, prefetch, superfetch and defrag most probably are better disabled under SSD.
    Reply
  • jimlocke - Wednesday, June 01, 2011 - link

    Pehu, I know this much after your posting, but I was curios what you ended up doing for swap.
    8GB of RAM almost seems like swap may not be needed, unless you have several memory-hogging apps open.
    Hope you still like your SSD. I'm looking at getting one soon, and agree this was an excellent article!
    -Jim
    Reply
  • krumme - Friday, October 09, 2009 - link

    First: I submit to the importance of random 4k for ssd.
    Second: Over the years I have highly valued the articles of Anand. It is remarkable to see such detailed and enthusiastic information.

    Now I have a few questions, following the general impact of this work.
    Some observations first:
    Following an article at Toms of a ssd article the 6 of September. The author was called a “Moron”, primarily as the random 4k synthetic bm was missing. The author was giving a different opinion on the indilinx vs intel, in the desktop sector, compared to Anand, giving more weight to transfer vs iops.
    In an discussion about a Kingston V-series review, one said that he would take the indilinx ssd any day because it was “750 times faster” – an argument based on iops.
    Another remark I have read several times is: “The Intel x25-M g2 is the only drive to get”.
    Another is: “I would like to buy the Dell xx, but it has an Samsung controller so its of no use”.

    I think it is time to stop, and make sure there is reason in what is happening for normal desktop use.

    Do we have blind test where to tell the difference between the Intel, Samsung and Indilinx?
    What is the actual real world bm fx. Win7 boot times for the 3 controlers?

    There is something called good enough. When is 4k random read/write time enough, to not notice any subjective improvement afterwards in win7? Could it be fx. 10M/s?

    The ssd is the best thing happening since 3d gfx, but I think we should enjoy what is happeing right now, because this time, could be the turning point where we soon are focusing on small differences.

    Anyone knows what´s the next big thing?
    Reply
  • bebby - Friday, October 30, 2009 - link

    Random 4k and its relevance for desktop use is really the main topic for me, too.
    If I assume that I only use the SSD for the OS and software and save my data on other, much less expensive HDDs, I doubt very much that this discussion is worth it. The Samsung SSD then suddenly looks not so bad at all and much cheaper...
    The next big thing for me would be an OS starting up in 5 seconds, like the OS we had in the 90s...making SSD obsolete.
    Reply
  • bebby - Friday, October 30, 2009 - link

    Random 4k and its relevance for desktop use is really the main topic for me, too.
    If I assume that I only use the SSD for the OS and software and save my data on other, much less expensive HDDs, I doubt very much that this discussion is worth it. The Samsung SSD then suddenly looks not so bad at all and much cheaper...
    The next big thing for me would be an OS starting up in 5 seconds, like the OS we had in the 90s...making SSD obsolete.
    Reply
  • marraco - Friday, August 13, 2010 - link

    I agree completely. I think that human beings can nottice the difference between a hard disk, and a non bad SSD, because the difference is too large, but over "good enough", it does not matter much if the SSD is 2X or 4X faster in 4Kb random R/W.

    But mine is just an opinion, and I don't have good data to test it. I would like to read an article with repeatable testing on human perception.
    Reply
  • SimesP - Wednesday, September 23, 2009 - link

    I haven't read all 254 comments (yet) but I'd like to add my thanks to everyone elses for the comprehensive and illuminating article. This, along with the previous AnandTech SSD articles have increased my understanding of SSD's immensely.

    Thanks again!
    Reply
  • ClemSnide - Friday, October 02, 2009 - link

    Anand,

    A couple of guys from HotHardware.com pointed me at your SSD article, and it allowed me to make an informed decision. Thanks!

    I wanted to speed up one game in particular (World of Warcraft) as well as routine OS tasks and web browsing. I think an SSD will do a bang-up job on at least the first two. The one I decided upon was the OCZ Agility 60 GB, which offers some growth room; I currently have 40 GB on my system drive. I know the Intel has better numbers, but I was able to get the OCZ for $156 after a rebate, which translates to decent performance at a price I can justify. (For the curious, it's available from TigerDirect for $184, and OCZ is giving a $30 rebate.)

    Even though my system build is still months away, this should be usable on my old clunker as well. Very nifty!
    Reply

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