Understanding Apple's Fusion Drive
by Anand Lal Shimpi on October 24, 2012 1:36 AM ESTDuring its iPad mini launch event today Apple updated many members of its Mac lineup. The 13-inch MacBook Pro, iMac and Mac mini all got updated today. For the iMac and Mac mini, Apple introduced a new feature that I honestly expected it to debut much earlier: Fusion Drive.
The idea is simple. Apple offers either solid state or mechanical HDD storage in its iMac and Mac mini. End users have to choose between performance or capacity/cost-per-GB. With Fusion Drive, Apple is attempting to offer the best of both worlds.
The new iMac and Mac mini can be outfitted with a Fusion Drive option that couples 128GB of NAND flash with either a 1TB or 3TB hard drive. The Fusion part comes in courtesy of Apple's software that takes the two independent drives and presents them to the user as a single volume. Originally I thought this might be SSD caching but after poking around the new iMacs and talking to Apple I have a better understanding of what's going on.
For starters, the 128GB of NAND is simply an SSD on a custom form factor PCB with the same connector that's used in the new MacBook Air and rMBP models. I would expect this SSD to use the same Toshiba or Samsung controllers we've seen in other Macs. The iMac I played with had a Samsung based SSD inside.
Total volume size is the sum of both parts. In the case of the 128GB + 1TB option, the total available storage is ~1.1TB. The same is true for the 128GB + 3TB option (~3.1TB total storage).
By default the OS and all preloaded applications are physically stored on the 128GB of NAND flash. But what happens when you go to write to the array?
With Fusion Drive enabled, Apple creates a 4GB write buffer on the NAND itself. Any writes that come in to the array hit this 4GB buffer first, which acts as sort of a write cache. Any additional writes cause the buffer to spill over to the hard disk. The idea here is that hopefully 4GB will be enough to accommodate any small file random writes which could otherwise significantly bog down performance. Having those writes buffer in NAND helps deliver SSD-like performance for light use workloads.
That 4GB write buffer is the only cache-like component to Apple's Fusion Drive. Everything else works as an OS directed pinning algorithm instead of an SSD cache. In other words, Mountain Lion will physically move frequently used files, data and entire applications to the 128GB of NAND Flash storage and move less frequently used items to the hard disk. The moves aren't committed until the copy is complete (meaning if you pull the plug on your machine while Fusion Drive is moving files around you shouldn't lose any data). After the copy is complete, the original is deleted and free space recovered.
After a few accesses Fusion Drive should be able to figure out if it needs to pull something new into NAND. The 128GB size is near ideal for most light client workloads, although I do suspect heavier users might be better served by something closer to 200GB.
There is no user interface for Fusion Drive management within OS X. Once the volume is created it cannot be broken through a standard OS X tool (although clever users should be able to find a way around that). I'm not sure what a Fusion Drive will look like under Boot Camp, it's entirely possible that Apple will put a Boot Camp partition on the HDD alone. OS X doesn't hide the fact that there are two physical drives in your system from you. A System Report generated on a Fusion Drive enabled Mac will show both drives connected via SATA.
The concept is interesting, at least for mainstream users. Power users will still get better performance (and reliability benefits) of going purely with solid state storage. Users who don't want to deal with managing data and applications across two different volumes are still the target for Fusion Drive (in other words, the ultra mainstream customer).
With a 128GB NAND component Fusion Drive could work reasonable well. We'll have to wait and see what happens when we get our hands on an iMac next month.
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Freakie - Wednesday, October 24, 2012 - link
Yep, if your SSD cops out then you're SOL. Which is why Intel's SRT copies the file instead of swapping it. Otherwise they're pretty much the exact same thing.ThreeDee912 - Wednesday, October 24, 2012 - link
Since it looks like Apple designed "Fusion Drive" to appear as a single drive to work seamlessly with existing software, I'm guessing things like Time Machine backup and Lion Recovery will still work fine. Even if the OS drive failed, a Time Machine drive has a built-in recovery partition so you can still restore your files if either drive fails.PaulRod - Wednesday, October 24, 2012 - link
"That 4GB write buffer is the only cache-like component to Apple's Fusion Drive. Everything else works as an OS directed pinning algorithm instead of an SSD cache. In other words, Mountain Lion will physically move frequently used files, data and entire applications to the 128GB of NAND Flash storage and move less frequently used items to the hard disk"That first bit is essentially a hybrid drive like the Seagate Momentus, the rest is exactly like Intel's ssd caching... infact that's all it is with maybe a tweak algorithm.
mavere - Wednesday, October 24, 2012 - link
Intel's SRT is still a caching technology, which is more like the first part of this "Fusion" thing.I can't think of any consumer products that would automate the transfer of the physical files themselves.
Freakie - Wednesday, October 24, 2012 - link
Nope, the first part is write-caching while SRT is mostly file caching (you can write-cache as well, if you like)epobirs - Wednesday, October 24, 2012 - link
I find myself wondering if operating at the file level is any real improvement. The end effect is the same and it shouldn't matter is all or part of an application is cached. If something isn't cached it's because it doesn't get used much. Why waste SSD space on it?If I use Word a lot but never do a mail merge, it isn't going to affect Word's performance for me if the files for the mail merge function live on the platter drive instead of the SSD.
'OS directed pinning operation' Isn't that what most modern desktop OSes do with frequently launched apps, cache them to RAM for quicker access? Why is that caching but Fusion isn't?
EnzoFX - Wednesday, October 24, 2012 - link
I thought the concept wasn't worth it as per the last podcast =P. I want caching to be great, until we have 2TB SSD that are affordable, there will be a benefit to not have to deal with managing two drives. When you install a bunch of stuff, it can be tedious to manage.Oh, and remember to check if the new 27" can still be used as a monitor as well please!
epobirs - Wednesday, October 24, 2012 - link
To say that this isn't SSD caching is purely a semantic argument. The only difference is that this works at the file level rather than the sector level. Other than that it is still a continuing popularity contest to determine which items are worthy of a place on the SSD or are just as well off on the platter drive.The big difference between this and building a PC with SRT enabled is that the Fusion drives comes pre-imaged with the Mac OS and bundled apps on the SSD, so you get the performance boost immediately instead of having teach it over the course of usage what files need to be cached. I expect PC OEMs to do the same as big brand machines start to offer SRT as a factory option.
slashbinslashbash - Wednesday, October 24, 2012 - link
No, the real difference that makes this not a "cache" is that the files do not live on both the SSD and the HDD. They only live on one or the other. A true "cache" would have all files residing on the HDD, with selected ones also cached to the SSD. (Similar to how a processor's L1/L2/L3 caches simply store the same stuff that's already in RAM.... it doesn't disappear from the RAM when it gets put into the cache.)epobirs - Wednesday, October 24, 2012 - link
Unless you look under the hood with System Report the user will only be aware of a single drive volume. This to mean screams "SSD CACHE!"Seriously, this is just building on SRT. Is there any doubt Intel shared their code with Apple to get started? End users get SSD benefits without having to think about what should go where. Where have I heard that pitch before?
I hope it's smart enough not to cache big video files regardless of how often they're played. That would be a horrible waste of SSD space for a type of data that gets little benefit from the medium. Though in a mobile setting it would be advantageous for battery life. Perhaps a control panel for such settings is needed.