SUMMARY
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Congratulations for digiKam, this is a proposal to improve it. I use it on Microsoft Windows to sync my photos loaded from iCloud Photos and OwnCloud Desktop, which use Virtual File System mounted on Windows. Currently, my system does not have enough space to hold all photos on disk, so only the photos I'm actually using are loaded on disk, while the rest only remains on cloud... I often use the option "Free Up Space" , that unloads photos from disk and only keeps them on cloud, but my issue is when I'm adding and syncing a Collection on digiKam, it loads the whole disk while syncing, and  leaves no physical space in my disk.
I wonder if there's an existing solution to this, and my guess is that these use cases will increase more and more in the future (with more cloud usage with virtual file system), so it would be nice if there was be an option on Configurations -> Configure digiKam -> Collections -> FOURTH OPTION Collection On Virtual File Systems.
I currently use "Collection on Network Shares" but this doesn't prevent everything to be loaded... I expected that digiKam could sync, by loading each photo and then applying "Free Up Space" property again automatically, leaving on Cloud the things that were already on cloud, and keeping local the things that were already local, thus not increasing disk space during sync.
Best regards!


STEPS TO REPRODUCE
1. Mount a Virtual File System directory (from OwnCloud, iCloud, etc)
2. Go to Configurations -> Configure digiKam -> Collections -> Add to Collection in Network Share

OBSERVED RESULT

See disk space be consumed when every photo is loaded to disk during sync

EXPECTED RESULT

Disk space should not change (or just slightly change) when photos are loaded during sync.

SOFTWARE/OS VERSIONS
Windows: 10
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION

This will affect any other system that supports Virtual Files.