If you’ve been using Microsoft’s OneDrive online storage for a few years, you likely remember “smart files,” aka placeholders. It was one of the few features—arguably the only—that made Windows 8 tolerable. Smart files let you see all your OneDrive files and folders in the Windows File Explorer, but not have to actually sync them to your PC. Those offline files would "stream" to your PC only when you tried to open them. Thus, you could save space on your PC while still accessing the rest of your files in OneDrive as needed.
The technology is easy to describe, but apparently hard for Microsoft to implement in Windows 10.
Two years ago, Microsoft released a beta test version of Windows 10 that didn’t support smart files. Windows 10 still doesn’t have this capability despite the continued customer angst. Microsoft says it is coming back—one day.
As shown below, OneDrive does have selective sync. In both the OneDrive app and in File Explorer, you can choose which folders to sync. But only those files and their folders are available on your PC, and they take up space. In Windows 10, you can't have files and folders stream to your PC on demand, as smart files did.
But yesterday, Google announced it’s bringing the smart files capability to Google Drive under the name Drive File Stream. It will be available only to Windows and MacOS for G Suite subscribers enrolled in G Suite’s Early Access program; that is, it’s beta software.
Dropbox has a similar feature called Smart Sync, also in beta. Dropbox seems to be having some of the same difficulties as Microsoft: Smart Sync so far is not available for Windows 10, only Windows 7 and MacOS 10.9 Mavericks and later.
As to why Microsoft dropped smart files in the first place, the official explanation appeared in an Office blog post on Jan. 7, 2015:
In Windows 8.1 we introduced a third sync engine that supported placeholder files, an innovative capability that lets you access all the files you have stored in OneDrive while only using a fraction of the local storage space. Customers who use OneDrive extensively on small devices found this feature extremely useful. ...
In Windows 8.1 certain apps would occasionally fail to open files that were placeholders because the app didn’t know how to issue commands to download the file, or the download would time out due to bandwidth speed. We noticed that certain file operations (including copy, move, and delete) had a higher degree of failure when placeholders were utilized. …
As a result, we knew we had to step back and rethink our approach and figure out a way to provide the features that customers liked about placeholders, without the impact on reliability, and deliver them in a comprehensible way.
Microsoft stepped back, and back, and back ... and 26 months later we still don’t have smart files for OneDrive.
Discussion continues on the AskWoody Lounge.