Why does my write/read speed go down?

[gr8pics]

I got a imac 27 late 2015, got a 2TB fusion drive, 1.5TB free space.
Using Disk speed test, loading file 5gb, and runs continuously, it shows 600/1500 once or twice, but 9 out of 10 times it slows down to 160/160
Can someone explain why?Im running the test from the root of user folder.

OSX 10.12.4

 

[Malus120]

Because you have a fusion drive. A (2TB) Fusion Drive is by definition 2TB of HDD (which will max out around 160/160) and 128GB of Flash (that's the 600/1500 you're seeing) using Core Storage to appear to you (the user) as a single storage device.

I believe it is set up to always have at least around 4GB of flash free for incoming writes, but with 500GB already used, its quite possible that either you're computer isn't always keeping 5GB of of flash free, or that sometimes the test messes up and somehow ends up not running on the cache portion.

 

[nambuccaheadsau]

Try this.

Shut down your computer, leave it for a few minutes, and boot using the on/off button behind the iMac. Do not simply use the 'Restart' option as this will not work. Keep us posted?