10.8. Core Data, Spotlight, and Time Machine

One of the disadvantages of Core Data is that it doesn’t integrate well with Time Machine. Time Machine uses hard links to avoid the need to duplicate files that have not changed. This works very well with bundles. A Keynote presentation, for example, contains a single file for each image, which typically doesn’t change, and a compressed property list representing the presentation. When Time Machine runs a backup, it will increment the link count for the image resource files and make a new copy of the plist. For the average presentation, this is only a small amount of the total data, and so Time Machine is very efficient.

Core Data, in contrast, stores the entire object graph in a single file. This means that any changes to it will result in a complete new copy being produced for every change. This can quickly fill up the backup volume and also degrades the user experience slightly as Time Machine monopolizes the disk and slows down other disk accesses during the backup period.