I post this as potentially helpful information for this community. I hope you find it provides some insight that helps you.
You should read this if you are considering an SSD, or if your Mac is struggling to contain your Aperture Library, or if you are just curious.
My Aperture Library (I call it ApLib, since we are on first name terms now), is getting smaller as I look at it.
Here's what is happening.
I have an iMac with SSD + HDD on order, and plan to use the SSD for Aperture. The whole ~400GB ApLib won't fit on it, but I figured that if I did a "Relocate Masters", I could put the now smaller ApLib on the SSD and the Masters on the HDD.
OK, so I made a copy of the entire ApLib, from one external disk on my MBP to another. Took about 8 hours. Initially, Finder thought it would take upwards of 190 hours, but that was before he (or she) started to move the actual Masters. At that stage, Finder started to revise the time estimate downwards, and after two hours, was getting to be quite accurate at "about 6 hours to go".
I then opened the copy of the ApLib with Aperture and told it to Relocate Masters to another folder on the same external drive. That took quite some hours, but the result was an ApLib size of 84.28GB vs close to 400, and the Masters folder is 309GBs. I was a little disappointed that the ApLib did not end up smaller, but at least at 84 GBs, it would be small enough to place on the SSD.
For some reason, I left Aperture running overnight, and when I got back to my Mac this morning, I saw that the external disk was rather active. Seems that Aperture is doing "stuff" to the library. Activity Monitor says Aperture has used well over 9 hours of CPU time, and sees CPU busy for Finder plus Aperture at typically over 70%. Disk activity is 300 to 500 reads per second with a much smaller write activity.
Now for the odd bit.
The ApLib which had been 84.28GBs is now just under 65.5. And it's slowly shrinking as I sit here typing.
It seems that Aperture is performing some maintenance function that makes the ApLib smaller.
PS another observation is that when performing relocate masters, Aperture is smart enough not to perform an actual copy of the masters. Looks like it just adjusted the file system pointers.
If you want to split your library into Masters and "The Rest", do it initially on one disk. It will probably be quicker.
Leave Aperture open after the Relocate Masters to allow it to clean up the library. Wait until Aperture's activity ceases, and the ApLib has finished shrinking.