Speed of Backup

[Mike Kallies <mike.kallies@gmail.com>]

Hello,

We're doing nightly backups using Tarsnap, approximately 100G of data.
 The backups are taking a very long time, and the time has been
increasing steadily since early January.

Backups which would normally start around 9pm and finish around 5am,
now start at 9pm and finish around 2:30pm.   There is not a huge
amount of change in the files. (~1-2G or so per day)

I was looking at filtering on modification date/time, but this could
introduce a retention issue.  We're backing up daily sets and using
those sets to maintain 30 day retention.  If we skip files based on
modification date, we would lose those files from the backups if they
haven't been modified.

Is there a method to speed this up?  Maybe a retain-file-only
if-modification-date-unchanged option?

Thanks,

-Mike