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Comment by 0942v8653 | Hacker News Reader

I'm not sure about snapshots, TM is made to work by just copying the directory structure over, and each backup is a fully functioning hierarchy. But cloning is definitely a big deal.

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Time Machine uses hard links to create a duplicate directory structure without duplicating all the files themselves; only changed files need to be copied.

As I recall, HFS+ was explicitly modified to support directory hard links, which is less common in the Unix world, explicitly to support this feature.

TM also maintains a folder called /.MobileBackups to store temporary backups while your backup drive isn't connected. OS X also maintains /.fseventsd, a log of file system operations that TM can use to perform the next incremental, instead of having to compare each file for modifications.

it doesn't "just" copy the directory structure over each time, it creates the structure for each backup, any files that are changed get copied in, ones that haven't changed are just hardlinked to the existing copies.

A snapshot is a copy of the file (and its blocks) at a given point in time. Subsequent writes to it will happen to new blocks, leaving the ones connected to the snapshot undisturbed.

Well, consistency is important too in backups. So TM will probably make a snapshot and do the backup from there. Avoiding moved files from later dirs to already processed ones etc.

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