tape backups

Hi everybody,

I'm trying to find a good solution to making backups to tape, where I
want to define "good" as:

- easy to use, like pg_dumpall, BUT
- not in a single file, so I don't backup my entire database cluster
with every differential backup

As I understand my backup program (Bacula) if a file changes at all
between differential backups then it gets backed up again in its
entirety. That seems pretty reasonable. So now I'm trying to figure
out how to get my postgres dump to end up in files in such a way that
little change in data means few file changes. But if there's no
native tool to do that (and it seems like there isn't) then setting
up something like that sounds like it might be a pain, as would
restoring from it.

Am I going about this the wrong way? Would it just be easier to do a
full pg_dumpall for my full backups and then build up a list of WAL
files with each differential? How do other people do it?