From my understanding it has a 120gb ssd attached to it, so does this mean that once i have more than 120gb of files on my mac the HDD portion will kick into action? thanks for the help everyone
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How does the 2TB fusion drive work?
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Yes.
The Fusion drive works on the same principle as hybrid HDDs. They look like normal hard drives, but have some flash memory.
The idea is very simple and intuitive. SSDs are fast, so put data that's frequently read/written on the SSD. HDDs are slow but offer lots of room. Put infrequently accessed large data on the HDD.
Software monitors what data you access frequently (for example OS bootup, safari, messages, desktop background, etc.) and dynamically moves it from the HDD to the SSD and back, with an emphasis on using the SSD as much as possible.
As to how exactly this is achieved, apple has put out relatively little literature on their core fusion technology, and expects customers to hope and pray that it just works.
With fusion drives, I highly recommend having a backup, because you have two potential points of failure, where reconstruction of the data is a nightmare, if it's even possible, moreso than if you had just a single drive. This can be understood because if a regular HDD fails (no fusion drive), you still have the option of removing the platters and getting all the data back. With fusion drives, as I understand it, the data will be fragmented, and I don't know about any tools apple has provided to put it all back together.
The Fusion drive works on the same principle as hybrid HDDs. They look like normal hard drives, but have some flash memory.
The idea is very simple and intuitive. SSDs are fast, so put data that's frequently read/written on the SSD. HDDs are slow but offer lots of room. Put infrequently accessed large data on the HDD.
Software monitors what data you access frequently (for example OS bootup, safari, messages, desktop background, etc.) and dynamically moves it from the HDD to the SSD and back, with an emphasis on using the SSD as much as possible.
As to how exactly this is achieved, apple has put out relatively little literature on their core fusion technology, and expects customers to hope and pray that it just works.
With fusion drives, I highly recommend having a backup, because you have two potential points of failure, where reconstruction of the data is a nightmare, if it's even possible, moreso than if you had just a single drive. This can be understood because if a regular HDD fails (no fusion drive), you still have the option of removing the platters and getting all the data back. With fusion drives, as I understand it, the data will be fragmented, and I don't know about any tools apple has provided to put it all back together.
Anandtech has a a very good article that was written about the Fusion Drive when it was first introduced in 2012. If you’re curious, it is an interesting read.
Source: https://www.anandtech.com/show/6406/understanding-apples-fusion-drive
When you hit the HDD after the SSD is full you will know, it is only so good at swapping commonly used files in and out
I've been using a 3 TB Fusion drive since early 2014. I've never "felt" hitting the HDD. The best way I can put it is that it all averages out.
It's true that if a block of code/data is currently HDD-resident, the initial Read will be at HDD speed (and Fusion deals in blocks, not entire files or data structures, no different than the way RAM and the CPU handle things - code and data structures are modular). That initial Read results in a move to SSD, in anticipation of subsequent Reads/Writes, which all occur at SSD speed. Just as with RAM, when space is needed on the SSD for newly-read code/data, blocks that have gone unread for the longest period will be moved back to HDD.
There's no such thing as "swapping commonly used files in and out" - Fusion is not making decisions like, "Well, he uses this app a lot, so we'll make it SSD-resident." It's purely a statistical function - whatever is in current use will be moved to SSD, and if it then languishes there unused, eventually it will be moved back to HDD.
My Fusion drive has 128 GB SSD and 3 TB HDD. Compare that to the laptops out there that have a 128 GB SSD, period. If they can live with 128 GB, I certainly can manage. Essentially, most everything I'm likely to need frequently is loaded into SSD on a constant basis.
Thanks for all this useful information everyone! This Imac was a gift and I don't use much room as it is (sub 60gb) so its looking like the HDD portion will never kick in.
Are you sure about this? My understanding is that frequently accessed data is moved onto the SSD and not immediately removed.
That's not right. From Apple:
Presented as a single volume on your Mac, Fusion Drive automatically and dynamically moves frequently used files to Flash storage for quicker access, while infrequently used items move to the high-capacity hard disk. As a result, you enjoy shorter startup times and—as the system learns how you work—faster application launches and quicker file access.
Fusion Drive manages all of this automatically in the background. And it comes already set up, so you don't have to do anything to make it happen.
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I'm not sure why you say, "Nope." What I said is that the "decision" is statistically driven. To clarify, while a machine's "behavior" might be described in anthropomorphic terms, it's not sentient. Its underlying algorithms are designed to deliver a human-defined result. What the algorithms are likely to "learn" over time is not the names of our favorite apps, but that the cream naturally rises to the top - over time, more and more items will be called to Flash as "frequently used" and those that might have early on been identified as frequently-used may turn out, over time, as being less frequently needed. It seems like "learning" but it's really mechanical winnowing.
Even though you're quoting an Apple support article, I consider it to be marketing materials, rather than technical, to deliver the desired message to the layman. Read that quote again. Does "...so you don't have to do anything to make it happen" sound like a technical description?
I wish I was able to find them at the moment, but my understanding comes from viewing videos of the few WWDC sessions that discussed Fusion, back when it was new.
Maybe Apple's Fusion algorithms say, "count disk accesses from HDD and when it hits 'X', move to SSD," but I have my doubts. The more conservative the algorithm is regarding moves, the more likely the data will be in the wrong, slow storage over and over again.
I'm thinking that RAM may be the better measure of whether the data/code is or is not frequently needed. As long as the data/code remains RAM-resident, it doesn't have to be called from either HDD or SSD. So perhaps the algorithms are watching RAM and effectively deciding, "We keep hitting this in RAM/this has remained RAM-resident for a particular time span, so it's time to move it to SSD." That could happen well before the next time the data/code needs to be called from HDD. There would also be system performance benefits in waiting to do that move until the CPU has nothing better to do (but before the data/code has to be called again from HDD).
The notion that the entire OS would be SSD-resident seems an over-simplification. There's much in the OS (and even in frequently-used apps) that is rarely if ever called. While (for argument's sake) 4 GB of unused System files might not put in a dent in a 2 or 3 TB Fusion Drive's 128 GB of Flash, it would be a substantial waste in the 24 GB Flash drive of a 1 TB Fusion. Sure, for all intents and purposes, all of the OS that you're likely to use is going to be Flash-resident. Again, in the "It just works" world of Apple, a bit of over-simplification in documentation is not a bad thing, especially when it's a feature that presents no options or settings to the end user.
As I've said in many previous posts about Fusion, I have both an all-Flash iMac and a 3 TB Fusion Drive-eqiipped iMac. In day-to-day use, it's hard to distinguish the two, performance-wise. Both of them boot at around the same speed, both of them rarely make me wait for anything. So, no matter how the OS manages that Flash, it just works. With no intervention or decision-making on my part. I don't have to manually copy data files to Flash at the start of a project, or move them back to HDD when work is done. I don't have to check if I'm running low on Flash and decide what app(s) needs to be demoted to HDD. It's the kind of drudge work that computers are really good at, and easily distracted/forgetful humans are not.
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Fried wrote:
"The Fusion drive works on the same principle as hybrid HDDs. They look like normal hard drives, but have some flash memory."
A "fusion" drive is actually TWO drives inside the iMac.
- an SSD "blade" drive (can be either 32gb or 128gb)
- a platter-based hard drive (can be 1tb, 2tb, 3tb).
They are "melded together" by the OS into appearing as one drive -- hence, "fusion".
The SSD portion of the 1tb fusion drive is a little too small to work well over time.
The 2tb/3tb versions will work better for longer.
However, none of them are the equal of having a "straight" SSD inside...
"The Fusion drive works on the same principle as hybrid HDDs. They look like normal hard drives, but have some flash memory."
A "fusion" drive is actually TWO drives inside the iMac.
- an SSD "blade" drive (can be either 32gb or 128gb)
- a platter-based hard drive (can be 1tb, 2tb, 3tb).
They are "melded together" by the OS into appearing as one drive -- hence, "fusion".
The SSD portion of the 1tb fusion drive is a little too small to work well over time.
The 2tb/3tb versions will work better for longer.
However, none of them are the equal of having a "straight" SSD inside...
Hybrid drives do not work on the same principle. They are dumb storage meant to host mainly the OS and and at most a handful of apps on the tiny SSDs they typically have. The Fusion Drive is intelligent, constantly monitoring and moving most used data to the SSD and least used to the HDD, as you properly described. It actually works amazingly well, though no real substitute for a pure SSD system.
They are also two separate physical drives, like fisherrman correctly stated. The SSD resides in a blade slot on the motherboard. The hard drive is a normal hard drive.
This is also how hybrid drives work.
They are not dumb, they dynamically move data between the flash and the platter
No, they do not. They were created to simply allow for fast bootup time and the fast opening of a limited number of apps, since hybrid drives have always had small amounts of flash storage.
Yes, but the fusion drive works in the same way. It has more space to play with, but the idea is the same.
Apple’s documentation on the fusion drive is really limited -_-
Both hybrid drives and Fusion drives are forms of caching. In the case of hybrid drives, which are a combined hard disk and SSD in one unit, the caching is implemented by the drive itself (although the operating system can supply hints). So the most recently used blocks end up on the SSD portion so that they can be accessed more quickly. In the case of Apple's fusion drives, the SSD and the hard drive are separate units and the operating system itself manages where the disk blocks are kept. But, in either case, the end result is the same, the most frequently used blocks are stored on the SSD. Differences between fusion and hybrid drives comes down to the amount of flash memory and the efficiency of the cache management software.