Why SSDs Care About What You Write: Fragmentation & Write Combining

PC Perspective's Allyn Malventano is a smart dude, just read one of his articles to figure that out. He pieced together a big aspect of how the X25-M worked on his own, a major key to how to improve SSD performance.

You'll remember from the Anthology that SSDs get their high performance by being able to write to multiple flash die across multiple channels in parallel. This works very well for very large files since you can easily split the reads and writes across multiple die/channels.

Here we go to write a 128KB file, it's split up and written across multiple channels in our tiny mock SSD:

When we go to read the file, it's read across multiple channels and performance is once again, excellent.

Remember what we talked about before however: small file random read/write performance is actually what ends up being slowest on hard drives. It's what often happens on a PC and thus we run into a problem when performing such an IO. Here we go to write a 4KB file. The smallest size we can write is 4KB and thus it's not split up at all, it can only be written to a single channel:

As Alyn discovered, Intel and other manufacturers get around this issue by combining small writes into larger groups. Random writes rarely happen in a separated manner, they come in bursts with many at a time. A write combining controller will take a group of 4KB writes, arrange them in parallel, and then write them together at the same time.

This does wonders for improving random small file write performance, as everything completes as fast as a larger sequential write would. What it hurts is what happens when you overwrite data.

In the first example where we wrote a 128KB file, look what happens if we delete the file:

Entire blocks are invalidated. Every single LBA in these blocks will come back invalid and can quickly be cleaned.

Look at what happens in the second example. These 4KB fragments are unrelated, so when one is overwritten, the rest aren't. A few deletes and now we're left with this sort of a situation:

Ugh. These fragmented blocks are a pain to deal with. Try to write to it now and you have to do a read-modify-write. Without TRIM support, nearly every write to these blocks will require a read-modify-write and send write amplification through the roof. This is the downside of write combining.

Intel's controller does its best to recover from these situations. That's why its used random write performance is still very good. Samsung's controller isn't very good at recovering from these situations.

Now you can see why performing a sequential write over the span of the drive fixes a fragmented drive. It turns the overly fragmented case into one that's easy to deal with, hooray. You can also see why SSD degradation happens over time. You don't spend all day writing large sequential files to your disk. Instead you write a combination of random and sequential, large and small files to the disk.

The Cleaning Lady and Write Amplification A Wear Leveling Refresher: How Long Will My SSD Last?
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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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