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2017 iMac Pro: Flash Drive (SSD)
Related: 2013 Mac Pro, 2017 iMac 5K, 2017 iMac Pro, 4K and 5K displays, diglloydTools, iMac, iMac 5K, iMac Pro, Mac Pro, Macs, SSD, storage, Thunderbolt, Thunderbolt 3
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This page compares SSD of the iMac Pro to other Macs.
Sustained transfer speed vs transfer size
The 2017 iMac Pro impresses mightily with its write speed, but what matters most of all for most applications is read speed of 32K, 64K and 128K transfers and with small reads the iMac Pro underperforms its consumer sibling. Which in turn underperforms its 2015 predecessor.
Indeed, the iMac Pro 32K read performance is little different from the 12TB Seagate Helium hard drive and its 64K read performance is only moderately better than the hard drive. It is gobsmackingly laughingly inappropriate for a pro machine. Apple ought to go back to Samsung SSD modules instead of designing its own.
Look at how the 2017 iMac 5K SSD performs at 32/64/128K reads (green dashed line), and yet the 2017 iMac 5K SSD is itself slower than the 2015 iMac 5K SSD for small transfer sizes. Apparently an Apple design trend, in a disappointing direction.
All in all, very disappointing—for most uses reads are far more important than writes. There is nothing 'pro' about the small-transfer-size read performance in the iMac Pro.
APFS and HFS are neck and neck in speed, but HFS (macOS Extended) outperforms slightly for writes at small to medium sizes.
Sustained transfer speed vs iMac 5K, large transfers
The 2017 iMac Pro delivers outstanding write speed, but in a curious inversion in which writes are much faster than reads.
For large transfers (only, see graph above), the iMac Pro is the better performer.
Sustained transfer speed, large transfers
2017 iMac Pro: writes at 3333 MiB/sec and reads at 2812 MiB/sec.
This graph shows transfer speed across the entire drive capacity. Result were essentially identical with the 1TB and 2TB SSDs.
Write speed is outstanding and much faster than reads, which is a curious and unexpected inversion of the usual behavior: usually reads are faster than writes, see the 2017 iMac 5K SSD speed and others.
The results are disappointing in one way: I was hoping for performance in the 6GB/sec range since there are two SSD modules—a RID-0 stripe of two 2017 iMac 5K modules should deliver nearly 6GB/sec. But the small transfer size disappointment discussed earlier is much more important for many applications.
Recommended Items for iMac Pro and iMac 5K
See the recommendations page for details on why these items are recommended.