ok I can confirm now that issue is same with SFTP or FTPS. whatever the method, if backup is on distant server, customer will not see it.
@fabieng
Sorry for the late response, a bit busy the latest days.
I did some tests though........ and I think there is another more important issue at hand - you are encountering issues due to the backup method chosen.
That is, the root cause of your problems - disk shortages - is probably related to the setup of your backups.
I can see that you use incremental backup - this often causes backup files to be larger than necessary : just use full backups with lower levels of rotation.
I can imagine that there is this flaw in your structural setup of backups, with three areas that can cause potential issues :
1 - incremental backup on a per-domain or a per-subscription basis : as noted before, disk space required for incremental backups can be huge, (and)
2 - the presence of a full or incremental server-wide backup : potential double usage of disk space, (and)
3 - incremental backups in general : when losing one of the incremental backup files, it will become difficult or even impossible to restore the backup,
implying that it might be the case that you have to investigate whether the design structure of your backups has to be reviewed.
It can be recommended to only run a server-wide full backup locally and, in addition, to run per-domain / per-subscription full backups remotely.
Hope the above helps a bit.
Regards........