I am using rsync to copy ~10GB of small files. What happens if the files in the source directory are modified while rsync is running?
rsync first scans the files and builds a list. so once the file is listed for sync, rsync will sync the latest change of file. but if the file is not in the list of files to be synced, which was built before starting the sync operation, then it will not sync it.
What happens if the files in the source directory are modified while rsync is running?
That depends if they are modified before rsync is running as well. As @Sachin said if they are not modified before the transfer then modifications during the transfer won't be picked up, however: