I have an Leap 15.0 KVM host with many Leap 15.0 clients. All use btrfs as root
filesystem.
Some days round midnight, I experience a severe sluggishness of the system.
Turns out, all VMs start their regular btrfs maintenance tasks via systemd
timers at the same time, namely midnight. Although each VM starts the
btrfs-{balance,trim,scrub} with low priority (IOSchedulingClass=idle,
CPUSchedulingPolicy=idle), this does not consider that other VMs also started
their btrfs maintenance tasks. This imposes a high load on the disks and all
VMs are barely responsive.

There is a systemd timer option RandomizedDelaySec. If this was set to like 1
day for btrfs-scrub.timer (runs every month) and like 12h for other btrfs task
(they run every 5 days), the VMs would stagger their btrfs-maintenance task and
not clog the disks.

Maybe RandomizedDelaySec can be added to btrfs-{balance,trim,scrub}.timers