With Ceph recommendation, 1TB HDD will require 1GB RAM, Mars 400 only has 4GB RAM on each microserver, does it support HDD with a bigger size than 4TB?
Indeed, on Ceph documentation hardware recommendations, A general rule of thumb is ~1GB of RAM for 1TB of storage space, however, this rule is more applying to the centralized architecture on general x86 server, when the powerful server motherboard shall allocate RAM resource to different OSD daemon.
Mars 400 ARM server is with microserver architecture, with 1xhost vs. 1x OSD, and each OSD has its dedicated hardware resource including CPU, memory, onboard flash and disk, so we don't need to follow the general rule of 1GB of RAM for 1TB of storage space.
Today, Ambedded Mars 400 (or Mars 400SES) can support the 3.5"HDD up to 14TB/each.