I/O scheduling, iotune and difference between read_bytes_sec and read_bytes_sec_max in qemu/kvm
Issue
I want to throttle the I/O using iotune and I wondering what is the difference between read_bytes_sec and read_bytes_sec_max? https://libvirt.org/formatdomain.html describes these tunables as:
read_bytes_sec
The optional read_bytes_sec element is the read throughput limit in bytes per second.
and
read_bytes_sec_max
The optional read_bytes_sec_max element is the maximum read throughput limit in bytes per second.
I understand it like read_bytes_sec is soft limit (can be breached when there are enough resources) and read_bytes_sec_max is hard limit which can never be breached. Is this assumption correct? What is the mechanism of evaluation if the disk is overloaded? Can the read_bytes_sec be trusted?
Environment
Red Hat Enterprise Linux 6
Red Hat Enterprise Linux 7
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