TimeVault

TimeVault is a simple front-end for making snapshots of a set of directories. Snapshots are a copy of a directory structure or file at a certain point in time. Restore functionality is integrated into Nautilus - previous versions of a file or directory that has a snapshot can be accessed by examining the properties and selecting the 'Previous Versions' tab.

Snapshots are protected from accidental deletion or modification since they are read-only by default. The super-user can delete intermediate snapshots to save space, but files and directories that existed before or after the deletion will still be accessible.

A snapshot is a copy of a directory at a certain point in time. Snapshots don't use space for the files that haven't changed but instead simply add a link to the same data on disk. When a file (link) is deleted, the link count for the data on disk is decremented. When it reaches 0 the data is marked free in the allocation table and new data can then be written to those blocks. As most allocations of data have only 1 file that links to them data is usually freed when its file is deleted.

Only files are hard-linked this way. In most filesystems directory hardlinks are disallowed because they can cause endless recursion. Apple specifically modified their system by adding mandatory recursion detection to prevent infinite loops, ostensibly purely for their backup software.

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