I was using aperture with referenced images. The aperture library is about 20gig, the folder containing the referenced images is 200gig. I exported versions of a selection of my pictures (6000) and imported those into photos.app for a trial run. The settings (checked) in photos.app are: iCloud Photo Library, Optimized Mac Storage, My Photo Stream, iCloud Photo Sharing.


Right now my Photos Library has a size of 23gig. Which is used mainly for Masters as seen below:

enter image description here

When checking my iCloud-Setting (in System Preferences) the size of photos in the cloud is 16gig.

enter image description here

I use a MacBookAir(late2010) right now with 128gig of storage. So I was hoping photos.app would upload all the pictures in full-res to the cloud and keep low-res pictures locally. Right now it seems the local copy is even larger. I had my mac book on wifi (and connected to power) for some days now (only 78 pictures left for upload).

So my questions are:

  1. What is the target size Photos.app is optimizing for?
  2. Is there any way to change/adapt this target size?
  • Edit № 1: When I dug through the Masters folder in the Photos Library package I realised that I saw pictures which I had deleted. And after checking File/Show Recently Deleted I realised that a large number of test import images I deleted prior of the import of my Aperture exports have been sitting there. After clicking Delete all the size of the library reduced to 14gig. Resulting in 23gig of free space which is quite close to the 20% of free storage range @bmike suggested as an optimiser target.
    – rul30
    May 4, 2015 at 19:51

5 Answers 5


So after some time (learning python on how to monitor and plot library size and disk usage) this is what I have observed regarding my first question. As written by @will-mallard and @bmike the optimizer targes for percentage free disk space. As shown below: the size of the picture library increases and decreases together with my disk usage. During those three days I did not use my mac book air for work. I only added pictures two times. first

Over a longer time scale, one can see the reduction of picture library disc space over time while I was working on my macbook. full

My followup question is now: What triggers the optimization? At first I thought it is some kind of free space threshold. But as it is easily seen in the last plot, something at the 24th May triggered a reduction of library space. I am not sure what that was.

  • 1
    Throw your scripts up on github? I bet we can find a case like your May 24 and look over the system logs for clues.
    – bmike
    Nov 6, 2016 at 0:16

Create a partition to the photo library dedicated, if you create a new photo library in there, and the maximum size is limited to the partition size. Of the 500GB of system drive, are using divided into partitions for the photo library of 50GB (10%).

I have a picture of 200GB on icloud, are used without any problems with the above settings.

  • 3
    Good Hack! As it kind if 'sets' the maximum size.
    – rul30
    Feb 29, 2016 at 8:20

I practice, the optimize space has zero effect on your photo library on OS X initially.

When you enable a system Photos library and opt in to the iCloud Photo storage, it tells the Photos Agent background process to start uploading photos to the cloud. When that initial pass is done, then deduplication in the cloud and download of new photos starts.

Assuming your Mac has room for the original photos - nothing gets optimized. Also, nothing gets optimized until the original is stored in iCloud.

Once your Mac runs into a storage issue on the volume where the Photos files are stored, it will start to optimize the local copies and reduce the amount of space needed so that the drive can have free space for other files.

I haven't seen Apple document these low storage space limits and I also haven't determined it empirically. Similarly, I don't have documented or empirical data on how far "optimization" will go and if at some point an error will be raised if photos cannot be compressed further.

I'm sure the code handles all of the above, but I don't have details on that.

As to your two specific items, it seems the software is smart enough to handle even your case where the library is large enough to have optimization happen before upload is finished.

I would say that I've seen it take 24 hours post upload to start reconciliation and download, so my guess is you might have to wait a while for the space savings to kick in as well as have Photos on a volume with low free space as an additional factor.

  • Thanks, I'll wait some days and we will see. My understanding was that OSX would need something like 10% of the disk space for swap-files. Having only 14gig free I thought the optimisation would kick in immediately.
    – rul30
    May 3, 2015 at 15:28
  • 2
    The paging system on 10.10 is much better at not reserving much swap. sudo du -sm /var/vm/* across 20 macs shows 2 GB the max swap to disk. Time Machine sets 20% and 10% free space limits for /Volumes/MobileBackups and purging of local Time Machine snapshots, so I'd guess Photos optimization would be somewhere between those two. You could allocate some non-sparse disk images or use dd to use up some disk space and play with things if you're adventurous. I'm curious to poke at it as well.
    – bmike
    May 3, 2015 at 16:56
  • very good idea. I'll try out once I am sure apple had enough time to 'optimise' the first time.
    – rul30
    May 3, 2015 at 17:34

After about 2 weeks of working with the Photos app on a Macbook Air 128GB, with a much larger photos library, I've noticed the following things.

There appears to be no fixed target size for the actual Photos library - instead its a target "free" space on my Macbook Air.

Currently, the Mac Optimisation appears to target roughly 10% free space on my Macbook (about 12GB)

Obviously you can only upload up to 12GB, but as soon as the import process begins, the Mac starts to re-allocate storage as it imports the photos to the Mac, and then begins uploading to iCloud.

It's a lengthy process importing 100's GBs of photos, but it should be possible.

My Photos library totals around 40GB on my MacBook Air

Hope that sheds some light... at least a little.


The size of your local Photos library is not necessarily indicative of the size of the photos stored on iCloud Photo Library. My local library is 83GB (of which 24GB is Masters) and the full-resolution uploads on iCloud Photo Library take up ~25GB.

Given that you still have some items to upload, and there could be discrepancies in how space is being reported (1000MB in a GB vs 1024MB in a GB etc), your Masters folder seems approximately correct versus what is reported in iCloud Photo Library.

  • @tubedoog, do you use the optimize mac storage option?
    – rul30
    May 3, 2015 at 15:07
  • No, but until it's finished uploading I doubt it will have deleted any originals from your computer to replace tennis with "optimized" versions.
    – tubedogg
    May 3, 2015 at 15:52

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