Odd performance drop with external SSD on new iMac Pro

MarckyG

macrumors member
Original poster
Jan 15, 2014
54
11
Germany
Hello professionals :D

I'm a proud owner of an iMac Pro for a few days now.
I'm coming from an iMac 2013 with Thunderbolt 1 and no retina display.

I observed an oddity with external drives and wanted to ask for advice...
It's with the SanDisk Extreme 900 (a 1,92TB SSD in Raid0).
While on my old Mac with Sierra and HFS+ the drive had normal speeds over USB3.0: around 420MB/s write and 450MB/s read.
Now with my new iMac, I was eager to test the actual speeds this drive can do with a USB-C Thunderbolt 3 connector...
250MB/s write and 850MB/s read... The USB 3.1 Gen2 protocol definitely let's the read speeds go to their full potential, but the write speeds suffer? And it's the same with USB-A cable or USB-C 3.1 Gen2 cable...
The read speeds go to around 400MB and after a second they drop to 300 and then stay at around 250MB/s...
I tested with Blackmagic Disk Speed and a file copying while watching the timer confirmed that...
Going back to my old iMac with HFS+ the write speeds are again better over the USB-A cable...
Does anyone have an idea? Is APFS at fault?
 

MarckyG

macrumors member
Original poster
Jan 15, 2014
54
11
Germany
Hey, they aren't for using as a boot drive, rather a media drive to work off. And diskutility doesn't give me the option to format to APFS... wouldn't the drive be incompatible with HFS Macs if I'd did anyway?
 

nambuccaheadsau

macrumors 68000
Oct 19, 2007
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Nambucca Heads Australia
Not suggesting they are boot drives. If you want to use with 2011 models and older machines yes leave HFS+. However the iMac Pro will have Mojave and if the 2013 model has an internal SSD and Mojave there will be no problems.
 

MarckyG

macrumors member
Original poster
Jan 15, 2014
54
11
Germany
The iMac Pro has an internal SSD Raid with High Sierra installed in APFS. SO you suggest formatting the Sandisk SSD in APFS would improve things. This means HFS+ and APFS SSDs don't really communicate well together? I wonder why...
Well interestingly enough, I can't format my external SSD in APFS since Disk Utility doesn't give me the option. If it isn's offered.... I feel like it's not a very safe thing to do even if there is a workaround?
 

mikehalloran

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Oct 14, 2018
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The Sillie Con Valley
Well interestingly enough, I can't format my external SSD in APFS since Disk Utility doesn't give me the option.
Yes you can. You just don't know how to do it. It's in the Edit menu at the top when in Disk Utility.
[doublepost=1544140380][/doublepost]
I use SSDs as externals and after formatting APFS and doing a fresh clone to each via SuperDuper, both are fest.
Why do you recommend cloning here? What does that accomplish?
 

MarckyG

macrumors member
Original poster
Jan 15, 2014
54
11
Germany
Well this has nothing to do with either booting or cloning...
Yes you can. You just don't know how to do it. It's in the Edit menu at the top when in Disk Utility.
I know how that works, and I know it's in the Edit menu (and other ways like right click also.
I just somehow formatted my external drive to Master Boot Record. That was why the APFS option wasn't there. I changed that now and formatted the drive for APFS, and the write speeds are still slow, like before. So it has nothing to do with the drive not being APFS...
 

MarckyG

macrumors member
Original poster
Jan 15, 2014
54
11
Germany

mikehalloran

macrumors 68000
Oct 14, 2018
1,750
453
The Sillie Con Valley
I know how that works, and I know it's in the Edit menu (and other ways like right click also.
I just somehow formatted my external drive to Master Boot Record. That was why the APFS option wasn't there. I changed that now and formatted the drive for APFS, and the write speeds are still slow, like before. So it has nothing to do with the drive not being APFS...
I never said it did. In fact, it doesn't.

I was eager to test the actual speeds this drive can do with a USB-C Thunderbolt 3 connector...
Your Sandisk is not a TB drive. You should connect it to the USB 3 port.
 

MarckyG

macrumors member
Original poster
Jan 15, 2014
54
11
Germany
If you're trying to help, I appreciate it. But I somehow don't understand what you mean...
I should NOT connect that drive to my USB 3 port. Why should I? It's a USB 3.1 Gen2 drive, capable of speeds over 800MB/s, which I get for the read speeds. USB 3.0 can only go up to approx. 500MB/s so... why should I connect it to USB instead of the Thunderbolt port (which is the only possibility of getting USB 3.1 Gen2 on Mac, the others are old USB ports)
 

jeboles

macrumors member
Dec 7, 2011
34
16
I wonder if the performance degrading your noticing is related to TRIM not working. Without TRIM, it would make sense that writes would suffer horrible over time. TRIM doesn't usually work with RAID modes.
 
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