This is a short one — I was using rsync to back up about a terabyte of data, which suffered a number of interruptions. Each time it restarted, I noticed it was copying over the same files over and over again. As it turns out, SMB rounds off the time stamps to the nearest two-second interval, so when rsync compares timestamps, it believes the file is different.
There’s a simple solution:
rsync -rlptDv –modify-window=1 [source] [destination]
The “–modify-window” switch tells rsync to relax its timestamp comparison just enough for it to behave.
The remaining switches are useful for backing up to a NAS box, it’s essentially -a (archive mode) expanded, but without preserving ownership or groups (which doesn’t work well on the NAS.)