stuff that gets skipped by TM is generally considered to be expendable
Not merely expendable, but in most cases stuff you wouldn't
want to restore.
- While a case might be made that log files might be useful for historical review, actually restoring a log file would constitute a loss of information.
- Restoring temporary files would only create orphans; the application that originally created them has forgotten them, and won't use them even if they're restored.
- A similar case can be made for cache files. In most cases the application that cached them has forgotten about them, and won't use them even if they're restored.
- Restoring your virtual memory file would be disastrous, even if possible.
- Your Spotlight database is actually a cache (in the sense that everything in it came from elsewhere and can be recreated if need be), but more importantly it's supposed to reflect what's on the disk volume now. Restoring your Spotlight database would break its contract, because you'd be setting it back to a reflection of what was on the disk volume then.
If it would be a mistake to restore a file, it's futile to back it up. Most of TM's exclusion rules are a consequence of this principle if futility, and not based on mere expendability.
It is true that some of the rules reflect expendability:
- A case could be made for restoring log files as part of a whole-disk recovery, even though you'd never restore individual logs, but not a strong enough case to justify the expense of backing them up over and over again. Who ever looks a log files anyway? (Present company excluded.) They're nearly expendable as is.
- Files in the trash have been explicitly marked as expendable by the user, but additionally they're probably redundant, having been backed up from wherever they were before they were put in the trash.
A few exclusion rules are for the sake of security. Backing up /Users/Guest would violate the contract that when Guest logs out all of its files are gone, gone, gone.
Some files that would be excluded for one of those other reasons could also be excluded for the sake of efficiency. Repeatedly backing up a large constantly changing file (like Outlook data) quickly becomes expensive enough, both in time and in the space it chews up in the backup, to make you feel that, even if the file is not expendable, hourly backups of it are. I think most user-defined exclusion rules are based on efficiency.
Of course, there's a lot of overlap. Your virtual memory file could never be restored, it's expendable, and backing it up would be a security breach as well as horrendously expensive. (It's also probably impossible to back it up even if you were so inclined.) A file that won't ever be restored must be expendable in some sense. The expense of backing up a file needs to be weighed against the cost of rebuilding it (or even of losing it).
But when I review TM's built-in exclusions, the most common basis I see is futility, not expendability. Don't back up what you're not planning to restore.
