There is still confusion regarding the new iCloud linked “Photos” app on the Mac, vs what we had with “iPhoto”.
Apple support is pretty clear about what happens when “Photos” constructs it’s new photo library, but does not delve into the details of how that effects you the user, or how that is done. This has led to some of the computer news sites presenting some bad advice, and suggesting you delete your iPhoto Library to save space. Don’t do that!
Here is my advice;
Do let Apple Photos sync your iPhoto library. But don’t delete you iPhoto library to save space, because no additional space is consumed on your Mac.
Based on my experience, this is what I think is going on;
When you are setting up Photos and it tries to create a new iCloud library for the first time, it pops up a warning. That warning is attempting to let you know that it will create your new Photos Library by deleting the existing Photos Library (not the iPhoto Library). It will then ask you to allow it to populate this new Photos Library with a combination of your original iPhoto Library and photos currently stored in iCloud Photos.
Creating this new iCloud Photos linked Photos Library on you Mac could takes days, but runs a background task. When it is done, you new Photos library is effectively starts on the day it completes the sync process, and is separate from your original iPhoto library, even though it is still linked to those original photos.
So what is going on?;
OS X and UNIX operating systems in general allow for the creation of “synthetic links” in file directories. A “sn” (synthetic link) looks like a file in “Finder,” but is actually like a “wormhole” back to the original file. It acts on that file even though it appears in a new library (i.e. Photos Library). In your user account, the “Pictures” folder your iPhoto Library and Photos Library shows a size reflecting it’s content, but that is only for human consumption. The actual hard drive space consumed remains no larger than it is with only an iPhoto Library.
With this arrangement, when you use Photos to edit and delete images, the Photos Library size will change on your Mac and iPhone/iPad, but the original iPhoto Library size will not change.
Like wise if you now go back and use iPhoto to edit or delete images, the iPhoto Library size will change, but not the Photos Library on your Mac or iPhone/iPad.
The thing all this OS slight of hand does is produce a directory of photos that can then be synced upward to iCloud Photos for storage, without compromising the original iPhoto library. This new Photos Library can then be mixed with the photos from you iPhone/iPad’s without detriment to your original iPhoto library. So that original iPhoto Library is now your archive, safe from further modification, as long as you don’t delete it.
Macdaddy of Seattle
https://support.apple.com/en-us/HT204655 (This page may not display correctly on you Mac, but do on iPad/iPhone. (it’s and Apple issue, so they need to correct the page.))
https://support.apple.com/en-us/HT204476 (This page may not display correctly on you Mac, but do on iPad/iPhone. (it’s and Apple issue, so they need to correct the page.))