Alternate data streams look like normal files but with an appended identifier.

For example test.txt:stream1 is an alternate data stream of test.txt. Move or copy the file and the ADS goes with it.

They can be created like other files (“echo > test.txt:stream1”)

You can see them with “dir /r” at the command line.

You can even have an alternate data stream with no corresponding file. In my opinion this is what thumbs.db should have been.

Yes well you’re not wrong.

Although I use ext4.

For Linux, the equivalent is Extended Attributes, although they come with significant limitations.

Not sure what you’re talking about. “Locked”?

For whatever insane windows-y reason, having a thumbs.db file on a network share is one of those slower scenarios for me. Which is odd because you'd expect that to be the kind of situation where it's actually useful.