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I have just upgraded OS X and opened the new Photos app to import my Aperture library. Everything went well (Photos work fine) but I am surprised to see that I didn't loose 100Gb free space on my HDD.

My Aperture library is still there, and takes 100Gb+. When I read information on the new Photos library, it also says 100Gb+. However Daisy Disk shows the following report:

  • Aperture library: 117 Gb
  • Photos library: 15 Gb

Are there some "symlinks" involved in the background? Are photos being slowly copied (100% CPU usage for the last 5 hours): I should add that the migration is finished and I can use Photos fine.

I am afraid at what would happen if I decide to delete either one of those libraries. I also wonder what happens if I edit photos in Aperture or Photos. I expect those to be completely separated (duplicated libraries) but apparently it's not the case.

I don't mind HDD usage I have plenty of space.

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Photos does not actually move the files or duplicate them when importing from an existing iPhoto or Aperture library. It simply creates hard links to the files in their original location from the new Photos library.

See how Photos saves disk space on Apple's site.

More on hard links from Ars Technica:

A hard link is simply a reference to some data on disk. Think of a file as a combination of a name and a pointer to some data. Deleting a file really means deleting the name portion of that duo. When there are no more names pointing to a particular piece of data disk, then that disk space can be reused."

You can delete your Aperture library without any problems, however you will no longer be able to use Aperture (unless you set up a new library). Any changes made in your existing Aperture library will not appear in Photos, and vice-versa.

Basically the actual image exists somewhere on the disk, and there are two pointers to the image (one from the old library and one from the new library). So long as at least one of those pointers exists, the data is kept. But when the last pointer is removed (i.e. the old iPhoto library itself is removed, then the image is removed from the Photos library), the data is also removed.

  • Exactly what I needed to know! So I can expect that if I delete the Aperture library my Photos library will now show 100Gb+ right? By the way I'm just discussing details here but am I correct to assume that the links are actually Hard links: if it was a symlink then deleting the Aperture library would break Photos. – Matthieu Napoli Apr 10 '15 at 4:23
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    Nevermind I have checked manually (and learnt a lot about hard links on the way): I confirm that a photo A.NEF stored in the Photos library has a hard link count of 2, and is referenced in the Aperture library. So Both libraries point indeed to the same file using hard links! Awesome :) – Matthieu Napoli Apr 10 '15 at 4:28
  • I don't see how this allows you to delete your old iPhoto or Aperture lib, without that trashing the originals. – Tetsujin Apr 10 '15 at 6:14
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    @Tetsujin Through the magic of hard links. "A hard link is simply a reference to some data on disk. Think of a file as a combination of a name and a pointer to some data. Deleting a file really means deleting the name portion of that duo. When there are no more names pointing to a particular piece of data disk, then that disk space can be reused." – tubedogg Apr 10 '15 at 6:28
  • Basically the actual image exists somewhere on the disk, and there are two pointers to the image (one from the old library and one from the new library). So long as at least one of those pointers exists, the data is kept. But when the last pointer is removed (i.e. the iPhoto library itself is removed, then the image is removed from the Photos library), the data is also removed. – tubedogg Apr 10 '15 at 6:30
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While the correct answer above notes that this may save space on Your hard disk but I believe I am finding that it still DOUBLES the space on your backup server... and I believe this includes time machine backups.

I have a 1 terabyte OS drive which has 200+GB full. After my upgrade to 10.10 (and photos), I whacked my time machine backup so that I could start from scratch (not concerned about having many versions of each file at this point) and asked it to back up to the 1.5 Terabyte backup drive. "Not enough room".

In addition, Crashplan seems to need 24 more days after the upgrade to back up my system. It seems squarely focused on the photos library. My guess is that each file will be on the backup server TWICE!!!

the beauty of hard links is that programs that access the data don't know that there are not two separate files. And the beauty is also the problem. This move was not well thought out, and I believe we need to clearly note that users need to delete their iphotos library from the drive (and maybe the backups) to not double the backup space.

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    You can bet that most cloud backup services will not recognize hard links and therefore store the file twice. Time Machine does this as well. Carbon Copy Cloner, on the other hand, recognizes and preserves hard links. See superuser.com/questions/836515/… – Jay Allen Jun 1 '16 at 21:02

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