Since Apple released their Fusion Drive, there have been a lot blogs focused on how to make a DIY (Do it Yourself) Fusion Drive for non-Fusion-Drive-equipped macs, but very few blogs showing a Fusion Drive’s performance in action.
Apple’s description of Fusion Drive really makes it sound fantastic: having frequently accessed files automatically stored on the SSD while infrequently used files are kept on the HDD. As Apple describes Fusion Drive, “…That’s because frequently used items are kept at the ready on speedy flash storage, while infrequently accessed items go to the hard drive. The file transfers take place in the background, so you won’t even notice.” The other half of the performance benefit is that Fusion Drives maintain a 4GB buffer space on the SSD. This means files written to the Fusion Drive are written to the SSD first and then migrated to the HDD when the drive is idle.
This automated file management really piqued my interest and there has been talk of the automated file transfer not working. I wanted to put it to the test personally and see this file transfer in action.
The two test computers
A 13″ late 2011 MacBook Pro (MacBookPro8,1), which had a 60GB OWC Electra 6G SSD installed in the main bay and a Toshiba 750GB 7200RPM hard drive installed in the optical bay via the OWC Data Doubler. A Fusion Drive was created between these two drives by following the steps provided in our earlier blog post OS X 10.8.3 Now Offers Fusion Drive Possibilities For Non-Fusion Drive Equipped Macs.
I also tested with a late 2012 Mac mini with a factory-configured Fusion Drive. In the lower bay of the Mac mini was an Apple SSD SM128E, which is a 6G SSD. The upper bay of the Mac mini had a 5400RPM Apple HTS541010A9E662 hard drive.
The testing procedures
I wanted to perform a test that was not overly complicated.
I took a 102.9MB MPEG 4 video file and copied it to the Fusion Drive. This copy occurred while only the OS and applications were installed, taking up about 15-20GB total space. This was to make sure the file was copied to the SSD. I then took a disk read benchmark of the video file using AJA System Test, with the video frame size settings at 1920×1080 10-bit (NOT 10-bit RGB).
We are able to clearly see speeds over 120MB/s, which is the average speed of 7200RPM Hard Drives. We know that we are working on the SSD because of these speeds.
The next stage of my testing was to use DiskTester, which is part of Diglloyd Tools, to fill the Fusion Drive until the SSD was completely full. I used a free demo of iStat Menus 4 to watch for the writes to transition from the SSD to the HDD. Even though Disk Utility only shows a single Fusion volume, iStat Menus shows the individual drives and their read/writes.
Once this transition occurred I duplicated the video file using the Finder ‘duplicate’ command, not by transferring the file again from my external. This duplicate file had the same name as the original with “HDD” added at the end of it. Once this duplication had completed I stopped DiskTester’s fill. Having copied the video file, I now had two copies of the file: one on the SSD and an identical file located on the HDD to provide me another baseline for benchmarking. I was able to see a clear difference in speed versus the earlier AJA benchmark.
Now that I had a baseline for SSD and HDD performance, the next task was to get the Fusion Drive to demote the video file from the SSD to the HDD. Fusion Drives maintain a 4GB buffer space on the SSD, so I used DiskTester’s ‘fill volume’ command to fill up the SSD until I saw the HDD take over on the writes. Once DiskTester was stopped, iStat Menu would show the 4GB buffer being rebuilt by having files written to the HDD from the SSD. Once the 4GB buffer was rebuilt, I would perform the AJA benchmark on the SSD video file to see whether it was still on the SSD or if it had been moved to the HDD. I repeated this step until I saw the SSD video file give HDD-level performance in AJA System Test.
I performed this test a total of three times. It is important to remember that the MacBook Pro has a 60GB SSD while the Mac mini has a 128GB SSD. The MacBook Pro Fusion Drive demoted the SSD file once the total space used was 100GB in the first test, 120GB in the second test, and 115GB in the third test. The Mac mini demoted the file at 250GB in the first test, a puzzling 138GB in the second test, and 264GB in the third test.
Once the file had been demoted, I attempted to get the file to be promoted back to the SSD. Apple says that frequently accessed files are kept on the SSD while infrequently used files are kept on the HDD. My plan to get my video file to be promoted back onto the SSD was to simply loop the video file, which would be constantly reading the file from the HDD. For the first round of testing I looped the video file in QuickTime for 20 hours. I then stopped the video file and let the computer sit idle for an hour with sleep disabled, but no promotion occurred. I then put the computer to sleep for two hours, then woke it and let it sit idle for another hour, but still no promotion had occurred. Lastly, I shut the computer off for an hour then rebooted it and left it idle for an hour. Still no promotion!
On the second round of testing I began by deleting the DiskTester fill folders from the Fusion Drive. This reduced the total used space on the Fusion Drive down to 15-20GB. I again looped the video file for 20 hours and performed the same three tests of idleness, sleep, and shutdown to instigate the file to be promoted to the Fusion drive. No luck!
On the third round of testing I deleted the DiskTester fill folders to make sure the SSD had plenty of free space. I looped the same video file both in VLC Player and QuickTime for about 60 hours. I once again performed the same 3 tests of idleness, sleep, and shutdown. Still no promotion! AJA was producing the same 60-80MB/s benchmarks.
Sadly, on both the DIY Fusion Drive of the MacBook Pro and the factory original Fusion Drive of the Mac mini I could not get my video file to be promoted back onto the SSD! My overall experience with Fusion Drive was good. While I was using both computers they performed exactly as if they were booted from a standalone SSD. I firmly agree that Fusion Drive offers some great features for average users, however prosumers and professionals should stick with independent SSDs and HDDs for maximum performance and flexibility.