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10.7: Use local Time Machine as an instant safety net
Authored by: wjv on Jul 02, '12 12:08:28AM
I have a bad habit of not checking back on hints I've submitted!

crarko and others express concern about the disk space usage of local backups. Turns out this isn't a huge issue for two reasons: Firstly, hard links are used for local backups, so any file in a backup set that's identical to the current file on disk takes up (virtually) zero additional space. Secondly, Lion industriously and constantly prunes local backups. If you open the Console application and search the system log for mtmd (the mobile Time Machine snapshot daemon), you'll see it at work creating new snapshots and purging old ones.

Mobile snapshots are stored under /.MobileBackups. On my heavily utilised MacBook Pro, this directory is just under a gigabyte in size. Sure, that's not nothing, but on a modern hard drive it's not prohibitive either.

That said, one should probably keep in mind that very large files are not Time Machine-friendly, including large disk images or VM images. For the former, one should use a Time Machine-friendly format like a sparse bundle, and for the latter it's worth upgrading to new versions of VM environments that support non-monolithic disk images.

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