Tarsnap - Online backups for the truly paranoid

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Running Tarsnap on multiple machines with the same key

Some users find it desirable to run Tarsnap on multiple systems with the same key in order to improve the effectiveness of the deduplication. This is entirely possible, provided that:

  • You only run Tarsnap on one machine at once.
  • The machine running Tarsnap has a cache directory with an accurate view of the server state.

On general-purpose desktop computers, it is unlikely that there is enough shared data that this is worth synchronizing the cache directories. However, in some business or academic situations, there could be a great deal of data shared between different computers, making it worth the extra care and setup.

There are two methods: sharing the cache directory, and using --fsck. In both cases, you should copy the tarsnap key file from one machine to the other.

Shared cache directory

If you have a shared cache directory (e.g., via NFS or SMB), then that cache directory will naturally be up-to-date. We recommend that you use a lock file to ensure that only one machine tries to use this cache directory at once.

Using --fsck

Running tarsnap --fsck will reconstruct the cache directory in order to match the Tarsnap server state; you could therefore run this command immediately before performing a backup on each machine. However, --fsck needs to download all the metadata for each archive, which is ~0.1% of the total archive size. Depending on your internet connection (and CPU for processing the metadata), this could take a while.