How Fusion Drive Works

Apple introduced the Fusion Drive in their latest line of Mac Mini and iMac. The fusion drive brings the best of the two worlds - mechanical hard disk drives and the latest solid state drives and presents it to the user as a single drive. Until now, Apple users had to choose between the two - regular HDDs and the SSDs, but now; thanks to the latest technology update that brings the capacity of the standard HDD storage and the faster read/write speeds of the solid state drives as a single package. I began searching on the Internet for how the new fusion drive works and found some really interesting information on various sources. Let me share it with you with simple and easy to understand words.

First of all, the Fusion Drive is basically a combo of two separate drives presented as one. That means, Fusion Drive is a summation of two parts acting as one. That is the reason the actual storage size of the Fusion Drives is 1.1 TB (128 GB of SSD + 1 TB of HDD). So what magic happens when you enable the Fusion Drive and store some data to it?

<a href="https://www.anandtech.com/show/6406/understanding-apples-fusion-drive" target="_blank" rel="noopener noreferrer">Understanding Apple's Fusion Drive</a> finds out that Apple creates a buffer of 4 GB on the NAND drive (SSD). All the writes that come to the drive are routed to this buffer first. The additional writes would be routed to the regular HDD. Having a 4GB cache ensures that the operations are faster for the user. Apple expects the 4GB buffer to be enough for the regular operations and that ensures performance for the HDD.

OSX Mountain Lion has been optimised to physically move the most frequently used files to the SSD, while other files are transferred to the HDD. The movement of files from SSD to HDD and vice versa is important to. Mountain Lion won't commit the moves unless the file transfers are complete. That means, in case of power failure, there won't be any data loss. The OS will simply make copy of the file in another location and once copy is successfully created, only then the original is deleted and space is freed up.

It's important to note that OSX still recognises the Fusion drives as two drives connected to the machine via SATA. A system report will mention two drives instead of just one. Interesting concept, isn't it?

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  • Abhishek Rawal
    Abhishek Rawal

    @abhishek-fg9tRh • Oct 24, 2012

    Sounds similar like Smart Response Technology.
    But from what I read by digging forums that, Unlike Smart response technology which 'temporarily' stores cache in >64GB SSD, Fusion drives permanently moves the data, according to what's hot & what's not.Like which of the file's been frequently used.Also, it will be permanently Re-Moved from SSD if the file's no longer frequently used.
    This technology is called Automatic tiering.
    Nice ain't it ?

    Seems like Apple mixing up two technologies & modifying for same use as previous technology.

    But a question arises in my mind, Smart response technology by Intel is doing 'almost' same (SRT creates cache rather than moving entire files in SSD) then what's difference(or say advantage) in developing similar technology.
    A marketing strategy ?
    Like, Noobs will say "hey my iMac uses Fusion drive, no longer simple drive.With 75% brighter screen.Gotta buy it, Apple is awesome ".... lol
    I mean this is same technology Intel's been using(talking about SRT)& other LED manufacturer's are using(Brighter displays).Just the thing is they are not selling white colored monitor.
    Food for thought friends.

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  • manishks
    manishks

    @manishks-AZybhs • Oct 25, 2012

    4gb cache!! Buffer of this size is huge.!! How much cache memory is there in other devices?? I always thought that very less memory is used as cache!

    One question!
    If the cache memory itself is so much large then will it be useful? Cache holds most frequently used data. And if its so big then memory access time will be more. And to decrease memory access time from main memory cache is used. So,...
    Is having large cache memory really useful? Coz' it will take more time to fetch required data(required memory address) from larger memory(so more no of memory addresses)!

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  • Jeffrey Arulraj
    Jeffrey Arulraj

    @jeffrey-xA7lUP • Oct 26, 2012

    As you said #-Link-Snipped-# to big a Cache needs more response time but if cache size is increased it is obvious that the processor speed is also quadrupled to incorporate such a huge size

    PS generally what is used Giga Bytes or Giga Bits when it comes to cache memory

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