Dave Price writes in with a very practical concern about the slightly abstract way in which Apple links the original photos in an iPhoto Library with the same images for an imported Photos Library.
I understand that the Photos library utilizes hard links to photos in my iPhoto Library, but now I want to delete the iPhoto Library since I won’t be using it anymore. If I delete the iPhoto Library then will these hard links be linked to nothing, or will the actual photos from the iPhoto Library be copied into the Photos Library?
Dave refers to the way in which Apple reduced the storage requirements when upgrading from iPhoto to Photos. Rather than copy all your original files over, Apple opted to use hard links, a form of Unix filesystem record-keeping that doesn’t have a metaphor in the real world.
With a regular file, it exists in one place on a drive. If you want to reference it elsewhere, you might create an alias, which has a modified icon and and (by default) the word alias at the end of its name. This is a variant on a deeper Unix option called a symbolic link that works largely the same way.
An alias or symbolic link doesn’t duplicate the file and it doesn’t move it. The file exists in one place, and the links or aliases point to it from elsewhere as a convenience.
A hard link is something different. The file still exists in one place on the disk, but can have multiple references to that file that are independent of each other. So the original image file of a photo in iPhoto can be hard linked in Photos, and you could say that it doesn’t precisely exist in either place, but as a sort of abstract item that equally exists in both places. (Quantum entanglement is the closest thing we have in reality, and it doesn’t work yet at a macro scale.)
Deleting a hard link in one place leaves all the other references intact. When the number of hard links drops to just one, you’ve just got a file! No hard links at all. And deleting that one reference, the file itself, truly does throw the file in the trash. Thus, delete your iPhoto Library, and—ostensibly—you won’t delete any files shared by Photos through hard links.
Having said all that, please make a complete backup of both your iPhoto and Photos libraries before deleting the iPhoto Library. You should be able to toss it and lose nothing, but I’m not so blithe as to suggest you whistle while you’re emptying the trash and assume all is well.
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