I added a spare disk for temporary backup files, and I mounted it at /var/lib/psa/dumps
Plesk backup is working like this:
backup to local repository: Makes a complete copy of the domain and its databases to /var/lib/psa/dumps/.........
Backup to ftp repository: Makes a complete copy of the domain and its databases to /var/lib/psa/dumps/...... and then makes a tar.gz of it, and send it to the FTP of your choice. If the FTP transfer is successful, the local copy is deleted. Otherwise, the local copy is kept, and so is the tar.gz file.
I strongly recommend not zipping (makes a tar instead of a tar.gz) because zipping takes a hell out of the resources...
I also strongly recommend making your own script with pleskbackup CLI and using the switches you want. It is a lot more flexible and allow a better control of what you're doing.
I made my own script. It makes a reseller config backup for each reseller, and then a domain backup for each domain. I send each backup to its specific date named file, and then I send files with ncftp to a server, allowing me to get a verbose log of what is happening if the FTP tranfer fail.