If you are already paranoid about your backups, you will want to be able to check that the data really made it to the dvd (and wasn't subsequently overwritten.) The old standard way goes something like this...

• Write a backup archive (tar for example) to disk
• Copy the archive to the dvd
• Remount the dvd (so you can read the file that was written)
• Compare the disk copy byte by byte to the dvd copy

A more elaborate scheme that lets you test the stored backups later,

• Write the archive to disk
• Calculate a checksum of the archive with, say, md5sum or sha1sum
• Write the archive and the checksum to dvd
• Remount the dvd
• Calculate the checksum of the cd/dvd copy and compare to the previously calculated checksum

The temporary disk space to hold a copy of the backup may be an issue. It is possible to use "tee" to send one copy of the archive to the cd/dvd writing process and one to the checksum program. (You will need to create a named pipe to be the output of the "tee" process and the input of the checksum.)

If your clients are anything like the ones I've had in the past, you will also want to write the date, the time, the name of the system, and the userid running the backup, plus any other interesting information available. That way you can check that the disks were labeled and stored correctly. (What, the disk labelled "Tuesday, Metropolis" wasn't made on a Tuesday or in Metropolis? Again?)

Finally, if you aren't paranoid about your backups yet, just wait until you start testing them!

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