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Up to 1527MB/s sustained performance
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2017 iMac 5K: Flash Drive (SSD)

MPG recommends 64GB OWC memory (big savings!) in: 27" Retina 5K Display, 4.2 GHz Intel Core i7 (Skylake), 1TB Flash, 8GB memory, AMD Radeon Pro 580 Graphics Card (8GB).

View iMac 5K at B&H Photo and see MPG’s computer gear wishlist. Not sure which Mac to get or how to configure it? Consult with MPG.

Is the 2017 iMac 5K really faster than the 2015 model? Yes and no!

In a classic case of not assuming too much, MPG tested the 2017 iMac 5K SSD speed. Apple likes to quote peak speeds, but all is not well for the vast majority of applications, which perform the bulk of their disk I/O in 1MB or smaller chunks—even Photoshop CC 2017 (so far at least),

Sustained transfer speed vs transfer size

Both machines utilized the Apple 1TB SSD.

MPG repeated these tests with highly consistent results.

Big surprise! The late 2015 iMac 5K SSD outperforms the 2017 iMac 5K for the critically important transfer sizes up to 1MB, particularly for writes, where the 2017 model lags badly. Many if not most programs do not read or write data in chunks larger than 1MB, so these results mean that the 2017 iMac 5K might actually perform less well in I/O intensive situations.

Reading a mix of 217000 files of varying sizes (html files and JPEG) shows that the performance is slightly in favor of the 2015 iMac 5K with a mix of text files and large JPEGs, but about 40% faster on the 2015 iMac 5K with small files (html files of a few kilobytes). This argues that things like file copying, or searching/grepping, git, etc will be faster on the 2015 model. Thus the 2017 iMac 5K is substantially slower for everyday tasks that involve intensive I/O with smaller reads, in spite of the 40% greater speed for large reads. This is a disappointing result in light of the speed gains in CPU and GPU speed.

HOWEVER, the 2017 iMac 5K can still outperform in mixed tasks that are I/O intensive with a mix of file sizes and CPU usage.

The two machines perform about equally for 2MB transfers.

At 4MiB and larger transfers, the 2017 iMac 5K offers much higher performance, with write speed of the 2017 model as fast as the read speed on the 2015 model.

MiB = 1024 * 1024. Multiply by 1.048 for MB/sec.

Real world speeds through the file system using diglloydTools DiskTester.
disktester run-sequential-suite

Flash Drive (SSD) Sustained Transfer Speed for larger transfers
2017 iMac 5K vs 2015 iMac 5K

Sustained transfer speed, large transfers

Both machines utilized the Apple 1TB SSD.

The 2017 iMac 5K delivers an outstanding performance that is 40% faster for reads and 38% faster for writes. Few applications are even capable of taking advantage of this superlative performance.

2015 iMac 5K: writes at 1520 MiB/sec and reads at 2075 MiB/sec
2017 iMac 5K: writes at 2102 MiB/sec and reads at 2915 MiB/sec.

Real world speeds through the file system using diglloydTools DiskTester.
disktester run-sequential --test-size 8GB

MiB = 1024 * 1024. Multiply by 1.048 for MB/sec.

Flash Drive (SSD) Sustained Transfer Speed for large transfers
2017 iMac 5K vs 2015 iMac 5K
OWC Accelsior 4M2 PCIe SSD
6000 MB/sec!
Mac or PC.


Ideal for Lightroom, Photoshop, video.
Capacity up to 16TB!
OWC Thunderblade Thunderbolt 3 SSD

Blazing fast, up to 16TB.

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