Please stop the madness: The Mac's Fusion Drive isn't about caching

Please stop the madness: The Mac's Fusion Drive isn't about caching

Summary: The words "caching" and "Fusion Drive" keep being used in the same sentence, or in the same paragraph. This is all wrong, so please save that cache reference for another day and another product.


There was some hubbub surrounding the release of machines with Apple's Fusion Drive Core Storage-based management software. And much confusion. The word "caching" crept into the stories and posts. The caching label appears to be sticking and that is incorrect.

I ran a search for "Fusion Drive" and "caching" and  found plenty of recent stories that describe the Fusion Drive as managing caching for a performance boost. Not.

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Instead, it's an implementation of storage tiering. This was described in a non-Apple, server-market context in a Wednesday announcement by SSD technology vendor Enmotus.  The company is calling its automated data migration solution "micro-tiering."

"Let's clear up any remaining misconceptions that tiering and caching are synonymous," said Mark Peters, senior analyst at the Enterprise Strategy Group. "They are quite different: tiering essentially creates a virtual pool of available storage, and moves the most frequently accessed or more active data to the faster tier - which in Enmotus' case is flash, with the solid-state capacity being additive to the overall storage pool.

Tiering is unlike caching, because the latter simply creates a temporary copy of the most accessed or active data in the cache; with tiering, the SSD operates as primary storage with no requirement to 'flush' the cache, which means less overhead and lower wear on the solid-state.

So, not caching. Tiering.

Topics: Apple, Hardware, Operating Systems, Storage, Web development

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  • New fancy name

    For not so new technology. Check Seagate Momentus XT.
    • The Momentus series is NOT the same thing. At all.

      Period. Your stating so indicates that you simply have done no research on the topic. In fact, teh Momentus is much closer to an actual caching technology. I could explain in depth the difference, and how block-based (and it IS block-based, not file-based) tiering works, but a more concrete display is perhaps easier to get the point across.
      Question: How big is the Solid State portion of the Momentus?
      Answer: 8GB
      Question: How big is the HDD portion?
      Answer: 500GB/750GB depending on model

      Question: How big is the TOTAL drive, as reported to the OS?
      Answer: 500GB/750GB

      Now ask these same questions about a Fusion Drive. The difference in the answers will be illuminating. Or at least it should be.
  • Qualifications for Journalism

    "I ran a search for "Fusion Drive" and "caching" and found plenty of recent stories that describe the Fusion Drive as managing caching for a performance boost. Not."

    Apparently the only requirement to be a writer nowadays is the ability to use google and copy/paste other peoples work? Seriously, where's the information that actually makes this a worthwhile read? I just dont get why ZDnet won't hire actual writers to create original material rather than fill the Apple side of ZDnet with two guys that just collect quotes and re-wrap them into "articles"?
    • Too much credit

      Look again at the search results and strangely both appear to be zdnet stories. So you can take google out of your equation.
      Little Old Man
  • Correct but useless precision

    Technically speaking, you are correct : tiering is the very method used here, and it is different than caching.
    Ok. So What.
    How many users, especially Mac ones, did heard about tiering ever ? I didn't, while have been using computers during more than 25 years.
    At the contrary, cache is more familiar. Cache memory, cache is quicker than standard etc.
    The cache concept speaks to a wider audience, and it is close enough to tiering to perceive the method despite the wrong word. I don't think anyone mentioning cache for Fusion Drive actually believe the data is stored temporarily. They just mean about putting aside the most used data. This, and only this matters.

    Wrong word though, but that doesn't prevent correct understanding.
    Let the Nerds care about the correct one.
    • I was going ot let this go by, but ...

      I just can't. It's lazy indifference like this that's lowering society's general mental acuity. It could be called the "Not really right, but close enough syndrome". Now if you're 5% off of the truth, and you explain it to someone that way, and they smudge it up 5% to explain it to someone else and so on and so on, eventually, someone is only getting 25% of the true explanation - is 25% good enough? Not in my world.

      With todays Wikipedia, and the multitude of on-line blogs (which are not fact, but opinion, and are not subject to the same fact finding guidelines that news articles are), we are inundated with partial truths and half facts. Unfortunately, most people take what they read and hear as gospel, and we end up with a million mental zombies walking around believing in something that just isn't true ...

      It is not only our responsibility as members of the human race to ensure that we understand things as correctly as we can (by double checking sources, researching facts, and generally asking questions), it is the responsibility of members of the media (from professional news reporter to private blogger) to ensure that what they report is accurate and fully understood, and if they think even for a second that someone doesn't quite understand, to explain it better.

      Thank you David, for taking the time to set the record straight, and for preventing even just a few people from turning into mental zombies.

      • Conversely

        Improvements in reporting will only encourage the zombie experience for people. The more people trust, the less they questions.

        So I say lets do away with accuracy and instead deliberately throw in some lies and half-truths. Then we'll all be forced to do some research and not take at face value, everything we read.
        Little Old Man
        • Boy you are something

          I find it strange you encourage lies and half truths to encourage people to look for answers.

          If you think that will help the mentally dead it will be a very sad world.
        • You're one to talk


      With totally different end effects.
      If you don't know the difference, please don't post.
  • Marvell already does this

    Their hardware chips are on various controllers, including those from HighPoint. They call it CACHING, regardless of which mode of operation you use, which it can either be fast (collective storage - like Apple) or safe (copied - which doesn't degrade data when SSD failures occur).
    • This is the point of the article

      A cache is a completely separate and temporary storage pool where the most previous used (read or written) data is stored. This pool must be flushed and synchronized with the main pool otherwise the data is lost. -- Just because Marvell has different modes of caching doesn't mean it's the same thing as tiering. Temporary storage has been around a long time and is used in many systems. Marvell's chips may have a setting that allow the use of the cache as a temporary data store, meaning it's never flushed to main storage but wiped once the client is finished with it.

      Tiering is combining disparate storage pools into one pool and managing the data intelligently by priority. The higher the priority the further it is moved up to the fastest device in the pool.

      While they are both methods of achieving faster performance from storage, they are not the same thing. Telling people they are perpetuates ignorance.
    • No they do not

      But thanks for putting your ignorance on parade for all to see.
      Care to differ? Please answer my question above.