I've been using amdump manually to test Amanda, and am backing up a full disk's file system (/dev/sda). The first time its run it produces a report with several gigabytes worth of backup:
USAGE BY TAPE: Label Time Size % Nb VFDailySet01 0:08 3393.4 4.1 1
Repeating runs on the same tape after this backup, seems to produce very tiny backups:
On the same tape??? How did you force amanda to do that? I hope you realize that the second amdump run overwrites the tape. Ok. It's just for testing, but you can test a restore with this test...
USAGE BY TAPE: Label Time Size % Nb VFDailySet01 0:00 0.2 0.0 1
Does amanda only save 'changes' to files, or is something incorrect happening here?
Amanda does not do the backup. Amanda is the scheduler of backups, and runs the OS dump program or gnutar. And both these programs have a notion of "incremental": everything changed since last run. They both do this on whole files, not parts of files. Incrementals can be done in levels. Level 0 is the initial full dump. A level 1 is all files changed since the last level 0. A level 2 is all files changed since last level 1 etc.
One of the most important parameters to amanda is how often you want
to schedule a level 0 (full) dump. See "dumpcycle".
If you executed the two amdump runs one after another, then, yes,
the second run has much less to dump than the next, assuming your dumpcycle is greater than 0. If you specify "dumpcycle 0" than this
indicates to amanda to schedule a full dump allways.
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