Thunderbolt SSD on iMac : Curious results...

[Alain Ternet]

I'm looking to buy a MacStudio soon but in the meantime I've already purchased a WD Black SN850X NVME Gen 4 SSD with a Thunderbolt external enclosure (Acasis TBU405PROM1) for video editing. For now I have an iMac 27" Retina 5K 2017 3.4 GHz with a 2TB fusion drive.

I did a BlackMagic test on my iMac and I got: 550 MB/S reading and 2000 MB/S writing
With the new Thunderbolt SSD I get: 2400 MB/S writing and 2375 MB/S reading
So far everything seems ok.

Then I tested copying a 2GB file from my iMac to the SSD and it takes 11.09 Sec.

But when I copy the same file from my iMac (from a folder copied to the desktop for example) it copies instantly, I don't even have time to start the timer. Why ? The Thunderbolt SSD is faster according to speed tests so how can we explain that?

 

[arw]

For drives formatted in APFS (Apple File System), when you copy a file on the same volume, it does not actually copy the file (bit per bit) but simply create a second "link" to the original data.
These few bytes are written almost instantly.

APFS clone files (‘copy on write clones’) are in between normal files and hard links. When you make a copy or duplicate of a file to the same volume, APFS creates a new file which shares the same data as the original. As the copy diverges, during editing, the changed data is saved separately, and the common data storage reduces until the files have no shared data at all.

Copy, move and clone files in APFS, a primer

What happens when you copy a large file to the same volume, or duplicate it, on the same APFS volume? Here are the answers.

eclecticlight.co

 

[Alain Ternet]

Thank you for your response, it makes more sense and explains my observation well.
So there's no real way to compare?

 

[arw]

Keep in mind, the 2 TB Fusion Drive is composed of a small 128 GB SSD and 2 TB HDD (spinning platter).
MacOS automatically manages which files are cached/stored on the SSD part and what is outsourced onto the large, slow HDD.
Therefore speed tests on a fusion drive can be very inconsistent by design.
A single SSD (even external) will outperform a Fusion Drive in almost every scenario.