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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  • jtleon - Tuesday, September 08, 2009 - link

    Yes I fell asleep atleast 3 times reading this article (it IS Monday afterall)

    Yes, Indilinx clearly rocks the SSD world - Now I know thanks to Anand!

    Stories like this set the standard for all review sites - I don't come away with the feeling I was just sold a bill of goods by some schiester in Intel's pocket, or otherwise.

    Great Job Anand! Keep them coming!
    Reply
  • SSDdaydreamer - Tuesday, September 08, 2009 - link

    I too am wondering whether TRIM will be available on the Intel Drives for Windows XP or Vista. I seriously doubt it, as the OCZ Wiper Tool appears to only be available for Indilinx controllers. Perhaps Intel will introduce their own wiper utility. I am leaning towards the OCZ Vertex or Patriot Torqx drives, as I am quite content with Windows XP and Windows Vista.
    I have an itchy trigger finger on these SSDs, but I want to hold back for the following unknowns.

    1. I would like to use the NTFS file system for my drive, but I am unsure of the proper/ideal block size.

    2. I would merely like to image my existing Windows Installation, but I am worried that performance or stability problems will arise from the NTFS file system. A fresh install could be in order, but it is preferred to image.

    3. Is there a way to change the size of the spare area? Maybe I have the wrong idea (perhaps only format part of the drive, unformatted space goes appends to the spare area?) I am willing to sacrifice some of the usable partition space for an increased spare area for improved performance.

    4. Are there complications with multiple partitions? If there are multiple partitions on the drive (for multi-boot) do they all share the same spare area? Is it possible to allow their own respective spare areas?


    Is there anybody out there that could enlighten me? I'm sure others would do well to have the answers as well. If I make any discoveries, I will be sure to post them.
    Thanks in advance.
    Reply
  • bradhs - Monday, September 07, 2009 - link

    IS there a "Wiper" app for Intel X-25m G2 drives? For people who don't have Windows 7 (TRIM) and want to keep the Intel X-25m G2 running smooth. Reply
  • smjohns - Tuesday, September 08, 2009 - link

    No there is no wiper tool for Intel drives at the mo. In addition to this the current firmware on the Intel drives do not have TRIM enabled. I guess this will be released soon after Windows7 is released. I think I have read somewhere that Intel are working on a TRIM version of it's Matrix Storage Manager software that will provide this functionality to the other operating systems. Reply
  • Burny - Monday, September 07, 2009 - link

    As many before me: great article! I learned a lot about SSD's. Even up to the point i'm ready to buy one.
    I still have 2 questions tough:

    2. Will TRIM be available on the G2 Intel drives for sure? Some sources doubt this: http://www.microsoft.com/communities/newsgroups/en...">http://www.microsoft.com/communities/ne...t=&l...


    3. As I understand, TRIM will work on a firmware level. That implies that TRIM will also function under Windows XP or any OS for that matter? Then why the need to build another TRIM into Windows 7? Or does a TRIM firmware enabled SSD simply allows the OS to use TRIM?

    Thanks!
    Reply
  • smegforbrain - Monday, September 07, 2009 - link

    While I consider myself handy with computers, I'm not the best technical mind when it comes to the details. You do an excellent job of presenting everything in a manner that it can be understood with little difficulty. I look forward to future articles about SSDs.

    I do have a question I'm hoping somebody can answer. I'm as interested in the long-term storage outlook of SSD drives as I am every day use. I've seen it said that an SSD drive should hold its charge for 10 years if not used, and it was discussed a bit earlier in this thread.

    Yet, none of my current mechanical hard drives are more than 3 years old; none of my burned DVDs/CDs are older than 5 years. It seems far more likely that I would replace an SSD for one with a greater storage capacity after 5 years tops than to expect one to be in use, even as archival storage, for as long as 10 years.

    So, is the 10 year 'lifespan' even going to be an issue with archival storage for most people?

    Will this worry over the life span of an SSD become even less of an issue as the technology matures over the next couple of years?
    Reply
  • Starcub - Tuesday, September 08, 2009 - link

    "So, is the 10 year 'lifespan' even going to be an issue with archival storage for most people?"

    No, but who takes wads of money out of their wallet to store it on their shelf?
    Reply
  • smegforbrain - Tuesday, September 08, 2009 - link

    "No, but who takes wads of money out of their wallet to store it on their shelf?"

    That is simply assuming that they will remain as expensive as they are now. They won't.
    Reply
  • BlackSphinx - Sunday, September 06, 2009 - link

    Hello! I'm taking the time to comment on this article, because I am very thankful for all of these awesome write-ups on SSD.

    I'm in the process of building an heavily overclocked i7 rig for gaming and video edition, and I was going to jam 2 Velociraptors in Raid0 in there. Why? I had only heard bad things about SSDs in the past.

    Reading your aticles, who are, while in depth, very clear and easy to understand, I understand much better what happened in early SSDs, what's so good about recent Indilinx and Intel SSD, and, truly, why I should forgo mechanical drives and instead go the SSD route (which, frankly, isn't more costly than a Raid0 raptor setup). In short, these articles are a great service to the end users just like myself, and if they were intended as such, you have passed with flying colors. Congratulations and thanks.
    Reply
  • Transisto - Sunday, September 06, 2009 - link

    Could someone reset my brain as to why there is no way to get a (very noticeable) improvement from USB thumb-drives. I mean these thing also get 0.1 ms latency.

    It's a bit extreme but for the same price I could get 9 cheap 8gb SLC usb drive for around 20$ each and put them in three separate PCI-USB add-on card (5$)

    They would saturate the USB controler with 3 drive in it so I Could get around 140mb/s read and 60mb/s write.

    Say you manage to merge that into a raid or ... ? Is eboost or Readyboost any good at scaling up ?

    Reply

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