But How About Incompressible Data and TRIM?

I mentioned earlier that TRIM has never functioned properly in SandForce SSDs when the drive is tortured with incompressible datam, which has never been a strength of SandForce. When it faces some, it's not exactly sure what to do with it. Your data will of course be written just like compressible data, but when your whole design is based on the assumption that the data will be compressed on the fly, there are some design trade-offs when it comes to performance with incompressible data. SandForce has said that third generation controllers should bring vast improvements to incompressible performance but we have no concrete numbers as of yet.

To test how TRIM behaves with incompressible data, I filled the Force GS with incompressible data and then tortured it with incompressible 4KB random writes (100% LBA space, QD=32) for 60 minutes:

Corsair Force GS—Resiliency—AS SSD—6Gbps
  Read Speed Write Speed
Firmware 5.0.2 5.0.3 5.0.2 5.0.3
Clean 494.1MB/s 507.6MB/s 270.5MB/s 266.8MB/s
After Torture 372.3MB/s 501.1MB/s 74.9MB/s 156.2MB/s
After TRIM 479.8MB/s 506.0MB/s 220.2MB/s 150.3MB/s

With firmware 5.0.2, both read and write speed degrade when tortured. The read speed doesn't degrade as much as write speed, but there is still a clear drop in performance. Fortunately TRIM will mostly restore read performance so there doesn't seem to be a similar problem as with compressible data. Write performance, on the other hand, restores but not fully. After TRIM write performance is about 81% of clean state performance, which isn't bad but not ideal either.

Firmware 5.0.3 seems to bring some changes to how incompressible data is dealt with. TRIM still doesn't work properly but as I've said before, I believe it's the way how the controller and firmware were designed, meaning that there isn't really a way to fix it. The good news is that write speed doesn't degrade nearly as much after torture as it did with firmware 5.0.2. Read speed also stays on-par with clean state performance. On the other hand, TRIM doesn't restore performance at all. As a matter of fact TRIM actually degrades write speed slightly but the difference is small enough to not raise any real concern. We did experience similar behavior with HD Tach, though.


SSD performance is all about trade-offs. As you improve one area, you generally weaken another. For example, you can opt for a large logical block size and get super fast sequential write speeds. The flip side is that random write speed will be horrible. Another good example is SandForce. They have chosen to concentrate on performance with compressible data, which has resulted in a trade-off with incompressible data performance.

Since it's generally impossible to have everything in one package, creating a good firmware and SSD is all about finding the balance. SandForce's approach in firmware 5.0.3 is in the right direction but it's far from perfect. TRIM now restores read speed after torture but in exchange, write speed takes a hit. I'm more satisfied with this behavior because the degradation in write speed is smaller and it seems that sequential writes and idle time will help to restore performance back to clean state. With firmware 5.0.2, read speed degraded for good; TRIMing the drive again and running HD Tach for several times didn't show any improvement.

What I'm more worried about is the TRIM behavior with incompressible data. With 5.0.2, TRIM at least worked somewhat as performance was better after TRIM than after torture. Sure, write speed doesn't go as low as it did with 5.0.2 but since most SSDs are used in TRIM-supported environments, I would rather take worse worst-case performance and partially working TRIM.

Hopefully SandForce will be able to find the right balance in a future firmware, which would be working TRIM regardless of the nature of data. 5.0.3 is a good start, but I feel that it concentrates too much on fixing one problem and as a result creates a bunch of new ones.

Firmware 5.0.3 to the Rescue The Corsair Force GS
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  • Sivar - Saturday, November 24, 2012 - link

    Do you understand how data deduplication works?
    This is a rhetorical question. Those who have read your comments know the answer.
    Please read the Wikipedia article on data deduplication, or some other source, before making further comments.
  • JellyRoll - Saturday, November 24, 2012 - link

    I am repeating the comments above for you, since you referenced the Wiki I would kindly suggest that you might have a look at it yourself before commenting further.
    "the intent of storage-based data deduplication is to inspect large volumes of data and identify large sections – such as entire files or large sections of files – that are identical, in order to store only one copy of it."
    This happens without any regard to whether data is compressible or not.
    If you have two matching sets of data, be they incompressible or not, they would be subject to deduplicatioin. It would merely require mapping to the same LBA addresses.
    For instance, if you have two files that consist of largely incompressible data, but they are still carbon copies of each, they are still subject to data deduplication.
  • 'nar - Monday, November 26, 2012 - link

    You contradict yourself dude. You are regurgitating the words, but their meaning isn't sinking in. If you have two sets of incompressible data, then you have just made it compressible, ie. 2=1

    When the drive is hammered with incompressible data, there is only one set of data. If there were two or more sets of identical data then it would be compressible. De-duplication is a form of compression. If you have incompressible data, it cannot be de-duped.

    Write amplification improvements come from compression, as in 2 files=1 file. Write less, lower amplification. Compressible data exhibits this, but incompressible data cannot because no two files are identical. Write amp is still high with incompressible data like everyone else. Your conclusion is backwards. De-duplication can only be applied on compressible data.

    The previous article that Anand himself wrote suggested dedupe, it did not state that it was used, as that was not divulged. Either way, dedupe is similar to compression, hence the description. Although vague, it's the best we got from Sandforce to describe what they do.

    What Sandforce uses is speculation anyhow, since it deals with trade secrets. If you really want to know you will have to ask Sandforce yourself. Good luck with that. :)
  • JellyRoll - Tuesday, November 27, 2012 - link

    If you were to write 100 exact copies of a file, with each file consisting of incompressible data and 100MB in size, deduplication would only write ONE file, and link back to it repeatedly. The other 99 instances of the same file would not be repeatedly written.
    That is the very essence of deduplication.
    SandForce processors do not exhibit this characteristic, be it 100 files or even only two similar files.
    Of course SandForce doesn't disclose their methods, but full on terming it dedupe is misleading at best.
  • extide - Wednesday, November 28, 2012 - link

    DeDuplication IS a form of compression dude. Period!!
  • FunnyTrace - Wednesday, November 28, 2012 - link

    SandForce presumably uses some sort of differential information update. When a block is modified, you find the difference between the old data and the new data. If the difference is small, you can just encode it over a smaller number of bits in the flash page. If you do the difference encoding, you cannot gc the old data unless you reassemble and rewrite the new data to a different location.

    Difference encoding requires more time (extra read, processing, etc). So, you must not do it when the write buffer is close to full. You can always choose whether or not you do differential encoding.

    It is definitely not deduplication. You can think of it as compression.

    A while back my prof and some of my labmates tried to guess their "DuraWrite" (*rolls eyes*) technology and this is the best guess have come up with. We didn't have the resources to reverse engineer their drive. We only surveyed published literature (papers, patents, presentations).

    Oh, and here's their patent: http://www.google.com/patents/US20120054415
  • JellyRoll - Friday, November 30, 2012 - link

    Thanks funnytrace, i had a strong suspicion that it was data differencing. In the linked patent document it lists this 44 times. Maybe that many repetitions will sink in for some who still believe it is deduplication?
    Also, here is a link to data differencing for those that wish to learn..
    Radoslav Danilak is listed as the inventor, not surprising i believe he was SandForce employee #2. He is now running Skyera, he is an excellent speaker btw.
  • extide - Saturday, November 24, 2012 - link

    It's no different than SAN's and ZFS and other enterprise level storage solutions doing block level de-duplication. It's not magic, and it's not complicated. Why is it so hard to believe? I mean, you are correct that the drive has no idea what bytes go to what file, but it doesn't have to. As long as the controller sends the same data back to the host for a given read on an lba as the host sent to write, it's all gravvy. It doesnt matter what ends up on the flash,.
  • JellyRoll - Saturday, November 24, 2012 - link

    Aboslutely correct. However, they have much more powerful processors. You are talking about a very low wattage processor that cannot handle deduplication on this scale. SandForce also does not make the statement that they actually DO deduplication.
  • FunBunny2 - Saturday, November 24, 2012 - link

    here: http://thessdreview.com/daily-news/latest-buzz/ken...

    "Speaking specifically on SF-powered drives, Kent is keen to illustrate that the SF approach to real time compression/deduplication gives several key advantages."

    Kent being the LSI guy.

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