Best choice for off-site backup: dd vs tar

Question

I have two 1TB single-partition hard disks configured as RAID1, of which I would like to make an off-site backup on a third disk, which I am still to buy. The idea is to store the backup at a relative's house, considerably far away from my place, in the hope that all the information will be safe in the case of a global thermonuclear apocalypse.

Of course, this backup would be well encrypted. What I still have to decide is whether I am going to simply tar the entire partition or, instead, use dd to create an image of the disks. Is there any non-trivial difference between these two approaches that I could be overlooking? This off-site backup would be updated no more than two or three times a year, in the best of the cases, so performance should not be a factor to be pondered at all.

What, and why, would you use if you were me? dd, tar, or a third option?

Answer 1

Between the 2, tar would be the winner in my book because of the portability that robertpostill mentions. If I were to do something like that on my own though, I would use rsync.

You mention performance not being an issue. I don't know what connection speed you have between the 2 sites, but if you are doing a remote backup and assuming a 512KBps cable modem up-speed you are looking at nearly a month to transfer the full 1TB. Which is why I would take a local copy first (via rsync) then move your disk to the remote host. Subsequent rsyncs will only copy the files that have changed.

Answer 2

For my money I'd do tar, the reason is that a tar archive is more portable than a dd archive. I'd be thinking that you would get some new hardware and then use a rescue O/S to get your disks set up and get the tar archive onto the new hardware. You know that tar will work on a rescue O/S and that even if you use a BSD-based rescue O/S (assuming you're using Linux as your primary O/S) you can still use tar and be confident of the result. dd is much less likely to work on another O/S.

Also you can twin tar with a compression algorithm like gzip or bz2 to get better space utilization.

Answer 3

I would prefer tar over dd for the purposes you're describing. However, I would recommend dar over tar because it can compress each file individually AND it supports incremental updates.

Answer 4

Why not use rsync, rsnapshot, rdiff-backup or something else that does syncing. That way you don't have to wait for the whole 1TB to copy each time.