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
Management Granularity
Much of Apple’s marketing on Fusion Drive talks about moving data at the file and application level, but in reality data can be moved between the SSD and HDD portions in 128KB blocks.
Ars actually confirmed this a while ago, but I wanted to see for myself. Using fs_usage I got to see the inner workings of Apple's Fusion Drive. Data is moved between drives in 128KB blocks, likely determined by frequency of use of those blocks. Since client workloads tend to be fairly sequential (or pseudo-random at worst) in nature, it's a safe bet that if you're accessing a single LBA within a 128KB block that you're actually going to be accessing more LBAs in the same space. The migration process seems to happen mostly during idle periods, although I have seen some movement between drives during light IO.
What’s very interesting is just how quickly the migration is triggered after a transfer occurs. As soon as file copy/creation, application launch or other IO activity completes, there’s immediate back and forth between the SSD and HDD. As you fill up the Fusion Drive, the amount of data moved between the SSD and HDD shrinks considerably. Over time I suspect this is what should happen. Infrequently accessed data should settle on the hard drive and what really matters will stay on the SSD. Apple being less aggressive about evicting data from the SSD as the Fusion Drive fills up makes sense.
The migration process itself is pretty simple with data being marked for promotion/demotion, it being physically copied to the new storage device and only then is it moved. In the event of a power failure during migration there shouldn't be any data loss caused by the Fusion Drive, it looks like only after two copies of the 128KB block are in place is the source block removed. Apple told me as much last year, but it's good to see it for myself.
By moving data in 128KB blocks between the HDD and SSD, Apple enjoys the side benefit of partially defragmenting the SSD with all writes to it. Even though the Fusion Drive will prefer the SSD for all incoming writes (which can include smaller than 128KB, potentially random/pseudo-random writes), any migration from the HDD to the SSD happens as large block sequential writes, which will trigger a garbage collection/block recycling routine in cases of a heavily fragmented drive. Performance of the SSD can definitely degrade over time, but this helps keep it higher than it would otherwise given that the SSD is almost always running at full capacity and the recipient of all sorts of unrelated writes. As I mentioned earlier, I would’ve preferred a controller with more consistent IO latency or for Apple to set aside even more of the PM830’s NAND as spare area. I suspect cost was the deciding factor in sticking with the standard amount of overprovisioning.
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philipma1957 - Friday, January 18, 2013 - link
I purchased a mid range mini a Qaud 2.3 with just a 1tb hdd. I added a samsung 830 512gb ssd as the second drive. If you put the ssd in with no format and boot with an external drive you then go to disk utility the internal ssd and internal hdd are highlighted in red. The disk utility sees it as a broken fusion answer yes to fix it and in under a minute you have a 1.5tb fusion. then do an internet recovery to load mountain lion and you have a standalone mac mini with a killer 1.5tb fusion. Everything you mention in your tests(about the ssd being taxed) is less true basically because the ssd is huge and in a 1 to 2 ratio with the hdd. This is the closest I have come to a 1.5 tb ssd.I copied a 500gb eyetv folder with 30 18gb recordings easy peasy. So if you have a 2012 or even a 2011 mac mini and mountain lion just add a big ssd and fusion away. Reply
hypopraxia - Friday, January 18, 2013 - link
I am with you on this one. Fusion drive with a higher SSD to HDD ratio nets huge gains in snappiness and overall SSD-ness of the fusion drive. I have only encountered IO slowdown once or maybe twice in the 2 months I've been running with Fusion. ReplyMaxComrie - Friday, January 18, 2013 - link
Love my job, since I've been bringing in $5600… I sit at home, music playing while I work in front of my new iMac that I got now that I'm making it online(Click on menu Home)http://goo.gl/xeJPd Reply
Samus - Saturday, January 19, 2013 - link
^^ SPAM ^^ ReplyMunkyman42 - Tuesday, March 05, 2013 - link
eeep!I wonder if this'll work on my 2010 Mac Mini Server. If I can squeeze another year or two or performance with even a 64GB SSD upgrade my wallet would be thrilled! Reply
Mr_SkoT_A - Tuesday, May 14, 2013 - link
I have done it on a late 2008 Unibody Macbook and an original Mini Server. Works like a charm! Replyhypopraxia - Friday, January 18, 2013 - link
So, after I read up on core storage, I rolled a fusion drive in my 2011 macbook pro 13", using a 240GB SSD (sand force controller) and the stock 320GB HDD. (I removed the optical drive. Stupid optical drive...) It honestly feels like a 550GB SSD. Then again, My ratio of SSD to HDD is 3:4, so as always, YMMV. Replykamsar - Friday, January 18, 2013 - link
Reliability-wise isn't Fusion Drive basically RAID 0? If it's doing block level migrations and one drive dies there's nothing left.Sure hope you've got time machine on... ;) Reply
pgp - Friday, January 18, 2013 - link
Yes, technically I think it's more similar to a JBOD configuration, but the reliability should be the same...IMHO Fusion Drive is good for noobs, but I'd rather choose which files should be stored in the flash drive and which ones in the mechanical drive, know about the free space in each disk, so I'd prefer a 128GB SSD and, separately, a 1TB hard disk to a 1.1TB Fusion Drive. Reply
TrackSmart - Friday, January 18, 2013 - link
I think drive configurations like this are really needed. Maybe not for you and I, but for 99.5% of people. Even people who aren't really "noobs".As an example, I purchased a 120GB SSD for a family member who is reasonably good with computers. It breathed new life into a 3 year old computer and was really noticed and appreciated. One year later, the whole thing was a disaster! There are documents, music, videos, etc all over the place. Usually 2 or 3 copies of the same files on both the SSD and the hard drive. Both nearly full. It took several hours to fix the mess.
Bottom line: Most people can't, aren't willing, or aren't well-organized enough to keep files segregated between drives. Even people who you probably think would be able to handle it by virtue of being reasonably computer literate. Reply