I'm talking about VM snapshots (and by transitivity, VDI snapshots). Retention is the number of backup you save on the backup repository, and it's totally unrelated to the number of snapshots when doing full or incremental backups.
I am using a Thinclient for my XCP Servers. Real small and cheap (if it is used) HP T620 with a 64G M2 SSD and 4 GRam Quadcore AMD CPU. To backup some VM i use a external USB3 HDD. It is running very nice and smoth. Power is between 5Watt an 20Watt excluding the USB Harddrive.
My coutumer had mostly two Servers and a Thin client.
my own Thinclient is used as a server for FHEM Homeautmatisation