1. Why perform a differential database backup as opposed to another log backup?
Speed of restores. When restoring a diff, you just need the latest one. When restoring logs, you need to restore all log backups since the full/diff.
It can be the difference between restoring 1 full, 1 diff and 10 log backups vs 1 full and 40 log backups for eg.
2. Why ever destroy the existing chain by performing a second full backup other than for purely space or management reasons?
Full backups don't break the log chain, so the question is moot.
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: Discussions on DB performance with occasional diversions into recoverability
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