@olivierlambert Remote storage - hard drive in raid 5

Yes

No need to update XCP-ng, we don't want to introduce a dedicated agent. It will rely on another mechanism. We'll publish about it before the end of this summer

Zstd is for XVA file format compression. There's no compression on delta, because we need to merge them on destination, when retention is reached (inside the backup repository). So it doesn't really worth it (since you'll have to decompress them to do the merge).