iMac (5K, Late2015) Fusion Drive question

[seveej]

Question, for those who know better (than me) how the Fusion Drive concept works IRL.

I'm working with a lot of fairly big Photoshop files (average file size 700-1400 megs), and the saving of these files (onto the 2,12 TB Fusion Drive) takes roughly 10-15 secs each.

Now I am assuming that the machine understands that the proper resting place for these files is on the HDD -part of the fusion Drive, but my question is: is MacOS optimizing the saving process by first saving them onto the SSD and then (when it can spare the time) transferring them to the HDD, or is it throwing them onto the HDD right away?

The question is relevant to me, as I will sooner or later need to expand the iMac's storage, and if the speeds/saving times I am now seeing are based on the machine saving the ≈1 gig PSD's straight onto the HDD, then I might splurge on a TB-SSD expansion...

TIA & RGDS,
Pekka

 

[Moonjumper]

I also have a late 2015 iMac with a Fusion drive. I think it gets sent direct to where it will be saved, although I'm not 100% on that. However, I believe macOS saves to the SSD according to the benefits of the file type and how regularly they are accessed. So PS files may get saved to the SSD, at least some of the time, then moved later as the space is needed for newer files.

An external SSD could be a good solution for you. It is likely to bypass your PS files ever needing to interact with an HDD. I use a USB3 external SSD for an extra installation for macOS (my Fusion has Mojave for 32-bit compatibility, such as Illustrator CS4, the external with a recent macOS for Xcode builds to the app store) and that runs nicely. An external USB SSD for you is likely to be an improvement, TB would be excellent.

 

[CoastalOR]

Here is some wikipedia information about fusion drives that might help.

Fusion Drive - Wikipedia

en.wikipedia.org

 

[petsk]

First to SSD. It's more difficult to say exactly when the files are moved to the HDD. Recent files are kept on the SSD.

Fusion drive will keep a 4 GB free cache on the SSD. So, if your SSD is full and you write a file larger than 4 GB it will slow down when it's writing the rest of the file on the HDD.

 

[JustMartin]

As petsk says, Fusion drives keep 4GB free for writes. So, a file save will go to the SSD first and the file will stay on the SSD until it is less frequently accessed than other things on the drive and will then move back to hard disk storage. I say file, because your work appears to be with discrete files, but Fusion works at a disk block level rather than a file one - so it's portions of files that are kept in the two locations

 

[seveej]

Thank you all for your comments thus far.
I think it would make sense for a fusion drive (SSD&HDD hybrid) to always first save every file onto the SSD, then use the spare capacity to shunt them over to the HDD. Therefore, I kind of assume that this is what happens.

Problem is, I'm not totally convinced. I've been looking at the saving times for various files and when I wave big files from PS (CS6), it pretty much looks that the effective saving time of a file is ≈130 megs/sec.

Considering that saving is more intensive than copying, that speed seems to be roughly on the outer limits of a 2015 2TB HDD, but it is far less than what I'd expect of an SSD.

Any thoughts?

 

[JustMartin]

Thank you all for your comments thus far.
I think it would make sense for a fusion drive (SSD&HDD hybrid) to always first save every file onto the SSD, then use the spare capacity to shunt them over to the HDD. Therefore, I kind of assume that this is what happens.

Problem is, I'm not totally convinced. I've been looking at the saving times for various files and when I wave big files from PS (CS6), it pretty much looks that the effective saving time of a file is ≈130 megs/sec.

Considering that saving is more intensive than copying, that speed seems to be roughly on the outer limits of a 2015 2TB HDD, but it is far less than what I'd expect of an SSD.

Any thoughts?

Perhaps a better test would be to time large file copies from external media to Fusion - timing PS adds one more layer of potential delay