Callers that pre-grow volumes on fill level currently have to list all
volumes of a collection and sum sizes themselves, which is expensive on
clusters with millions of volumes. The master already tracks each
writable volume's effective size in memory, so it can answer the
question directly.
Add grow_avg_fill_threshold to VolumeGrowRequest: when set (e.g. 0.6),
growth only happens once the layout's writable volumes have reached
that average fill ratio. A layout with no writable volumes skips the
check, since that is the case growth exists to fix. VolumeGrowResponse
now reports the observed avg_fill and writable_count whether or not
growth was triggered.
bruce-zzz
643338f9b5