[BWStearns]

I have file A that's in two places and I run this.

I modify A_0. Does this modify A_1 as well or just kind of reify the new state of A_0 while leaving A_1 untouched?

[madeofpalk]

It's called copy-on-write because when you modify A_0, it'll make a copy of the file if you write to it but not A_1.

https://en.wikipedia.org/wiki/Copy-on-write#In_computer_stor...

[bsimpson]

Which means if you actually edited those files, you might fill up your HD much more quickly than you expected.

But if you have the same 500MB of node_modules in each of your dozen projects, this might actually durably save some space.

[BWStearns]

Thanks for the clarification. I expected it worked like that but couldn't find it spelled out after a brief perusal of the docs.

[kdmtctl]

What will happen when the original file will be deleted? Often this handled by block reference counters, which just would be decreased. How APFS handles this? Is there any master/copy concepts or just block references?

[lgdskhglsa]

He's using the "copy on write" feature of the file system. So it should leave A_1 untouched, creating a new copy for A_0's modifications. More info: https://developer.apple.com/documentation/foundation/file_sy...