* volume.balance: rank by physical disk usage
* volume.balance: keep -byDiskUsage ranking on one scale across the fleet
A server that does not report disk bytes ranks at whole volume-equivalents
while reporting servers stay below 1.0, so during a rolling upgrade the
balancer drains nearly empty old-build servers onto physically fuller ones.
Decide the scale once: rank by physical used percent only when every server
reports disk bytes, otherwise fall back to the data-size ranking for all.
Normalizing the fallback by maxVolumeCount instead would reintroduce the
over-configured-maxVolumeCount distortion this flag exists to avoid.
---------
Co-authored-by: Chris Lu <chris.lu@gmail.com>
