Performance of MacBook Air SSD Targeted from iMac
So I tried it out and wanted to share my findings. First off, the equipment I was using:
- Early 2015 13" i7 MacBook Air, 8GB Ram, 500GB SSD
- Late 2015 27" i7 iMac 5K Retina, 16GB Ram, 1TB SSD
I restarted the Air in target disk mode & plugged it into the iMac via Thunderbolt 2. It wouldn't boot. After a few minute, it just showed a ban icon (circle with a line through it).
I ran some 5GB Blackmagic Speed Tests to gather transfer rates (in MB/S):
- iMac against its SSD - 1817 Read, 1510 Write
- Air against its SSD - 300 Read, 241 Write
- iMac & Air against USB3 Flash Drive - 245 Read, 185 Write
- iMac & Air against USB3 External Drive - 160 Read, 170 Write
- iMac using Air via Thunderbolt 2 shared folder - 400 Read, 490 Write
- iMac using Air in target disk mode - 79 Read, 107 Write
So obviously there's a big performance loss for target disk mode.
If you're wondering why I'd want to target the air, it's because I mainly work off my Air and don't want the hassle of an external drive, thumb drive, file syncing, etc. I love the idea of getting to my office and "docking into" a faster processor, more RAM, and a big screen.
In theory you should not loose too much speed. Most modern SSDs utilize SATA 6 Gbit/s SSD controllers which support 500 MB/s read/write speeds. Thunderbolt 2 works at a theoretical 20 Gbit/s. So Thunderbolt2 should not be the bottleneck.
However.. Macworld did a performance analysis in 2011. It was a test on Thunderbolt 1, but it should still might give you some insights. If you go to Macworld - More Thunderbolt speed results and scroll down to the section "Benchmarks: Target Disk Mode via Thunderbolt" you see that performance did take a huge hit. Even if you double the speed for Thunderbolt 2, you'd still loose over 50%.
Then there is the more recent (2014) The Instructable article High-Speed Data Transfers between Macs with Thunderbolt, which says:
Thunderbolt is capable of faster speeds though this upper limit is a combination of the maximum read/write speeds of the Mac’s SSD and that Parallels VMs are split into many smaller files. ↩
Sorry no definitive answer.. And either way I'd recommend against this setup. I use Dropbox and/or Google Drive to sync files between my machines. This works well for me.