On: Tarsnap – Online backups for the truly paranoid

[caivvoacmh]

Hi cperciva can you please explain the following:

1) "Every Tarsnap archive acts like it is completely independent of all other archives"

Lets say I'm backing up `/somedir` that looks like this:

# archive1 - initial backup

  ── somedir
      ├── file1.txt
      ├── file2.txt
      └── file3.txt

# archive 2 - 2nd backup

  ── somedir
      └── file3.txt (modified)

Are you suggesting that if I delete `archive1` and `rm -rf /somedir` that I can restore `/somedir` with `archive2` alone?

If so, how is that possible?

2) Does tarsnap stream the backup data to the tarsnap service or does it create the encrypted archive and only after this upload it?

For instance, if I've got 100 GB to backup (first time), do I need 100 GB of free space for tarsnap to work?

[cperciva → caivvoacmh]

Are you suggesting that if I delete `archive1` and `rm -rf /somedir` that I can restore `/somedir` with `archive2` alone?

If archive2 only contains somedir/file3.txt, then if you extract archive2 you'll get somedir/file3.txt but not file1.txt or file2.txt.

But if you create archive2 containing all three files, tarsnap will recognize the duplicate data in file1.txt and file2.txt and not re-upload it; but when you extract archive2 you'll get all three blocks. Tarsnap reference-counts blocks of data so that when you delete archive1 it only deletes the data which is not used by any of the remaining archives.

I often tell new Tarsnap users that they should start by forgetting everything they know about incremental backups. Tell Tarsnap what data you want to have in an archive, and let it do the work of figuring out what's new.

Does tarsnap stream the backup data to the tarsnap service or does it create the encrypted archive and only after this upload it?

Tarsnap uploads data as it collects it. Tarsnap needs a small amount of disk space to keep track of which blocks have been uploaded previously, but it's less than 1% of the size of the data archived.