@asltf The Xen grant table for IO is set too low. This was never an issue since the host machine always did host rather CPU intensive guest machines and less CPU intensive.

Mastodon and taking btrfs snapshots is rather I/O intensive. Grant table settings have to be set by boot parameter for the Xen hypervisor.

Yes, Xen. I used BSD jails and Xen in parallel between 2005 and 2010. I think now it's time to migrate to Linux/KVM.

Good old times, 3/2006:
https://cdprojekte.mattiasschlenker.de/Public/Artikel/PC-Magazin_Linux_2006_03_-_Grossrechnerfeeling_-_Xen_Aufbau_Grundlagen_Konfiguration.pdf