Usefulness of local copy

I just read the manual section on local copies (1.3.3) and find it lacking. It starts talking about large collections on NAS or external drives but then switches to working on images when traveling. So, I’m confused about the use case for local copies.

It appears to make a copy of an image into a temporary location. But the program needs access to the library to do this. So if you are traveling, and your library is at home, and you import new images you have just taken, how does making a local copy on your travel machine help you?

My situation is: Using a Windows 10 desktop monster machine with 3 screens, 64GB RAM, etc. But travel with a Windows 10 laptop. My main image library is on a USB 3.2 SSD, even when using the desktop machine. If I want to travel with the laptop I can take the USB drive with me. Will using local copies benefit me at all?

Yes, use local copy to work on the files you want without needing to have the external drive plugged in. When you’re done, sync back to the external.

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So you are saying I plug in the external drive, which gives darktable access to the library, I find the images I want to work on and make local copies, then I disconnect the external drive. Now I can work on the images that have local copies. When done I plug the external drive back in and have darktable sync the local copies. Is this accurately describing the use case that local copies was created to address? Or is there another scenario where it is also useful?

Yes, that is it.

Just some small questions to @paperdigits:
does that procedure require sidecars to be written (and checked on start-up)?
and would you suggest starting dt with the “-library :memory:” when using local copies in the described scenario?

@rvietor I’m not sure you can use local copies with :memory: library because it requires a library image to get started. In other words, how would the local copies be seen if you are looking at an empty :memory: library when darktable starts up?

Oops, I think I misread the scenario as dealing with two different computers, not one computer with a slow external disk…

I don’t think so, but I’m not 100% sure.

I dont think this would work, because the database tracks what is copied locally.