A Month with Apple's Fusion Drive
by Anand Lal Shimpi on January 18, 2013 9:30 AM EST- Posted in
- Storage
- SSDs
- SSD Caching
- Mac
- Apple
- Fusion Drive
Fusion Drive: Under the Hood
I took the 27-inch iMac out of the box and immediately went to work on Fusion Drive testing. I started filling the drive with a 128KB sequential write pass (queue depth of 1). Using iStat Menus 4 to actively monitor the state of both drives I noticed that only the SSD was receiving this initial write pass. The SSD was being written to at 322MB/s with no activity on the HDD.
After 117GB of writes the HDD took over, at speeds of roughly 133 - 175MB/s to begin with.
The initial test just confirmed that Fusion Drive is indeed spanning the capacity of both drives. The first 117GB ended up on the SSD and the remaining 1TB of writes went directly to the HDD. It also gave me the first indication of priority: Fusion Drive will try to write to the SSD first, assuming there's sufficient free space (more on this later).
Next up, I wanted to test random IO as this is ultimately where SSDs trump hard drives in performance and typically where SSD caching or hybrid hard drives fall short. I first tried the worst case scenario, a random write test that would span all logical block addresses. Given that the total capacity of the Fusion Drive is 1.1TB, how this test was handled would tell me a lot about how Apple maps LBAs (Logical Block Addresses) between the two drives.
The results were interesting and not unexpected. Both the SSD and HDD saw write activity, with more IOs obviously hitting the hard drive (which consumes a larger percentage of all available LBAs). The average 4KB (QD16) random write performance was around 0.51MB/s, it was constrained by the hard drive portion of the Fusion Drive setup.
After stopping the random write task however, there was immediate moving of data between the HDD and SSD. Since the LBAs were chosen at random, it's possible that some (identical or just spatially similar) addresses were picked more than once and those blocks were immediately marked for promotion to the SSD. This was my first experience with the Fusion Drive actively moving data between drives.
A full span random write test is a bit unfair for a consumer SSD, much less a hybrid SSD/HDD setup with roughly an 1:8 ratio of LBAs. To get an idea of how good Fusion Drive is at dealing with random IO I constrained the random write test to the first 8GB of LBAs.
The resulting performance was quite different. For the first pass, average performance was roughly 7 - 9MB/s, with most of the IO hitting the SSD and a smaller portion hitting the hard drive. After the 3 minute test, I waited while the Fusion Drive moved data around, then repeated it. For the second run, total performance jumped up to 21.9MB/s with more of the IO being moved to the SSD although the hard drive was still seeing writes.
In the shot to the left, most random writes are hitting the SSD but some are still going to the HDD, after some moving of data and remapping of LBAs nearly all random writes go to the SSD and performance is much higher
On the third attempt, nearly all random writes went to the SSD with performance peaking at 98MB/s and dropping to a minimum of 35MB/s as the SSD got more fragmented. This told me that Apple seems to dynamically map LBAs to the SSD based on frequency of access, a very pro-active approach to ensuring high performance. Ultimately this is a big difference between standard SSD caches and what Fusion Drive appears to be doing. Most SSD caches seem to work based on frequency of read access, whereas Fusion Drive appears to (at least partially) take into account what LBAs are frequently targeted for writes and mapping those to the SSD.
Note that subsequent random write tests produced very different results. As I filled up the Fusion Drive with more data and applications (~80% full of real data and applications), I never saw random write performance reach these levels again. After each run I'd see short periods where data would move around, but random IO hit the Fusion Drive in around an 7:1 ratio of HDD to SSD accesses. Given the capacity difference between the drives, this ratio makes a lot of sense. If you have a workload that is composed of a lot of random writes that span all available space, Fusion Drive isn't for you. Given that most such workloads are confined to the enterprise space, that shouldn't really be a concern here.
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zlandar - Friday, January 18, 2013 - link
"Really this seems like a solution for the lazy or technically naive."If everyone was technologically literate spam wouldn't exist and computer companies wouldn't need customer service for stupid questions. Reply
jeffkibuule - Friday, January 18, 2013 - link
Aren't a lot of solutions built for the technologically naive? ReplyNCM - Friday, January 18, 2013 - link
Apple's principal market, especially for the iMac, is to home and small business users. Once again dragging out the familiar, but still applicable, automotive metaphor, I'll point out that most people don't want to work on their cars. They just want to drive reliably to wherever they're going. That's the need that Apple's FD addresses, and it seems to do so rather well.Sure, the price adder is a bit higher than one might hope, but probably not so much that it'll frighten away prospective buyers.
Interestingly though, it lost our sale. I was ready to order another iMac with a 256GB SSD and a 1TB HD for the office. We keep most of the files on the server, but a 128GB SSD application/boot drive is a bit tight. However a 256GB SSD is just right, allowing plenty of free space to maintain SSD performance. The additional 1TB HD is then repurposed for local Time machine backup.
But that's not an option for the new iMac, which offers only HD or FD. And I'm not about to make a risky and warranty busting expedition into its innards in order to roll my own SSD solution (although my own MacBook Pro has a self-installed 512GB SSD).
Instead I ordered up a 256GB SSD Mac mini, plus what turned out to be a very nice 24" 16:10 IPS monitor from HP. Although I would have preferred the all-in-one iMac solution for a cleaner installation without gratuitously trailing cables, the Mac mini with SSD, i7 and 8GB RAM options is fast and effective. Reply
ThreeDee912 - Friday, January 18, 2013 - link
Wasn't this the kind of thing said about virtual memory in the 60's and 70's? Some people back then thought manually managing the location of everything in memory would make things more efficient, until some guys at IBM (or was it Bell Labs?) showed you saved heck of a lot more time letting the machine do it instead of trying to move things around yourself.This Fusion Drive really does reminds me of virtual memory. RAM and HDD mapped in a way so it appears as a single type of memory. Most stuff gets placed into RAM first, some stuff spills over onto the HDD, and stuff gets copied back and forth depending on how frequently it's used. The fast RAM is first priority, but there's the HDD as kind of a backup.
It's a bit different from a caching setup, where the computer has to "guess" a bit more about what should really be on the SSD. It's like the HDD is priority here, while the SSD is secondary.
And just like with virtual memory, none of this would matter if you had a huge amount of RAM or a very large SSD. Reply
web2dot0 - Saturday, January 19, 2013 - link
Great comment ThreeDee9. Someone with a rational mind.To all those "experts" who claim that it's better to manage it yourself, you can also write every program in ASM. It'll be fast and small, but I'll be done with the project in 1/10 the time. The point is .... the product is not meant to provide "absolutely the best possible configuration". It's meant to be best all around solution.
If you guys still don't get it. Well, I guess all these years in the education didn't really help you because logical people think rationally. Reply
psyq321 - Monday, January 21, 2013 - link
Hmm... is it just me who finds it slightly disturbing that we are comparing memory management (and, in some posts later, C vs. assembly coding) with the decision on how to organize documents/files?I would say that the intellectual investment is not really to compare.
Which does not mean that I have anything against SSD caching solutions - on the contrary, I see nothing wrong with ability to transparently manage the optimal location for the content. Reply
TrackSmart - Friday, January 18, 2013 - link
A month ago, I would have said the same thing, but see my other post to understand why more people need this than you think. The proportion of people who can handle manually segregating their files is much, much smaller than most of us realize. I have three systems setup with both an SSD and a HDD and have no troubles. But we are a tiny, tiny minority of users. ReplyBrooksT - Friday, January 18, 2013 - link
Excellent point and insight.I'm 40+ years old; I still know x86 assembly language and use Ethernet and IP protocol analyzers frequently. I'm fluent in god-knows how many programming languages and build my own desktops. I know perfectly well how to manage storage.
But why would I *want* to? I have a demanding day job in the technology field. I have a couple of hobbies outside of computers and am just generally very, very busy. If I can pay Apple (or anyone) a few hundred bucks to get 90% of the benefit I'd see from spending several hours a year doing this... why in the world would I want to do it myself?
The intersection of people who have the technical knowledge to manage their own SSD/HD setup, people who have the time to do it, and people who have the interest in doing it is *incredibly* tiny. Probably every single one of them is in this thread :) Reply
Death666Angel - Friday, January 18, 2013 - link
I wonder how you organize stuff right now? Even before I had more than one HDD I still had multiple partitions (one for system and one for media at the time), so that I could reinstall windows without having my media touched. And that media partition was segregated into photos, music, movies, documents etc. That is how I organize my files and know where what is located.I don't see any change to my behaviour with an SSD functioning as my system partition and the HDDs functioning as media partitions.
Do people just put everything on the desktop? How do you find anything? I just don't understand this at all. Reply
KitsuneKnight - Friday, January 18, 2013 - link
Do you not have any type of file that's both large, numerous, and demands high performance?I regularly work with Virtual Machines, with each of them usually being around 10 Gb (some being as small as 2, with the largest closer to 60). I have far too many to fit on my machine's SSD, but they're also /far/ faster when run from it.
So what do I have to do? I have to break my nice, clean hierarchy. I have a folder both on my SSD and on my eSATA RAID for them. The ones I'm actively working with the most I keep on the SSD, and the ones I'm not actively using on the HDD. Which means I also have to regularly move the VMs between the disks. This is /far/ from an ideal situation. It means I never know /exactly/ where any given VM is at any given moment.
On the other hand, it sounds like a Fusion Drive set up could handle the situation far better. If I hadn't worked with a VM in a while, there would be an initial slowdown, but eventually the most used parts would be promoted to the SSD (how fast depend on implementation details), resulting in very fast access. Also, since it isn't on a per-file level, the parts of the VM's drive that are rarely/never accessed won't be wasting space on the SSD... potentially allowing me to store more VMs on the SSD at any given moment, resulting in better performance.
So I have potentially better performance over all (either way, I doubt it's too far from a manual set up), zero maintenance overhead of shuffling files around, and not having to destroy my clean hierarchy (symlinks would mean more work for me and potentially more confusion).
VMs aren't the only thing I've done this way. Some apps I virtually never use I've moved over (breaking that hierarchy). I might have to start doing this with more things in the future.
Let me ask you this: Why do you think you'd do a better job managing the data than a computer? It should have no trouble figuring out what files are rarely accessed, and what are constantly accessed... and can move them around easier than you (do you plan on symlinking individual files? what about chunks of files?). Reply