AppFS has no problem with that. The problem comes from keeping different packages on different disks -- since it caches them all into the same directory by def

AppFS has no problem with that. The problem comes from keeping different packages on different disks -- since it caches them all into the same directory by default. It would be trivial to make it cache files from certain packages into different places as part of the configuration -- that's easy enough. The issues come into place for files that are shared (having the same SHA1) between packages, then you either have to duplicate the file or have some policy of searching for it. A better approach, if you care about this, is simply to have a lifecycle policy where more recently used blocks live on the SSD and less recently used blocks move to slower storage (bcache can do this for you), or are deleted (this is a planned feature: http://appfs.rkeene.org/web/tktview?name=930821d0fb http://appfs.rkeene.org/web/tktview?name=930821d0fb ).