rsync multiple filesystem to one big FS
 

If let's say there are more than two filesystem /fs1 /fs2 /fs3 and I want to rsync it to /fs_new which is a bigger capacity. Is it possible to rsync those without deleting the previously rsync files to the destination?

What exactly are you trying to accomplish? Which files should not be deleted (source or target)?

The only possible problem I see is if, for example, /fs1 and /fs2 both have the same directory structure and file names (/fs1/foo/fubar.txt and /fs2/foo/fubar.txt) If both are rsynced to /fs_new then you end up with one /fs_new/foo/fubar.txt

rsync does have options to prevent overwriting or overwrite in special conditions (size vs time).