I have a Ubuntu Server with 6 SSD raid 10, and one FreeNAS with 10 HDD in RAID 5, the space on the server are fully, i have only 10GB to start the services like sql, php, etc. and i want to all the data when he write on that to transfer automaticaly on NAS via FTP or some API.
I would suggest using something like
rsync to run at a interval like every 15-minutes to copy data from source, to target. You can use Rsync over SSH to copy the files directly, or mount the NAS box to the Ubuntu box by using NFS, SMB, or FTP.