About resetting usable space and how it is related to store usage calculation in Producer Flow Control

Issue

• During my broker starts, I can see below usable space calculation warning messages:

2020-05-17 05:41:08,142 | WARN  | AMQ-1-thread-1   | BrokerService                    | he.activemq.broker.BrokerService 2098 | 162 - org.apache.activemq.activemq-osgi - 5.11.0.redhat-630283 | Store limit is 307200 mb (current store usage is 352 mb). The data directory: /NFS/kahadb only has 163758 mb of usable space. - resetting to maximum available disk space: 163758 mb

• My producer application has been under flow control due to the usage reaches the store limit:

2020-05-20 09:45:02,567 | INFO  | 1.20:49020@61616 | Queue                            | mq.broker.region.BaseDestination  683 | 162 - org.apache.activemq.activemq-osgi - 5.11.0.redhat-630283 | Usage(default:store:queue://test-queue:store) percentUsage=92%, usage=172306321372, limit=171713555306, percentUsageMinDelta=1%;Parent:Usage(default:store) percentUsage=100%, usage=172306321372, limit=171713555306, percentUsageMinDelta=1%: Persistent store is Full, 100% of 171713555306. Stopping producer (ID:producer-1:1:1) to prevent flooding queue://test-queue. See http://activemq.apache.org/producer-flow-control.html for more info (blocking for: 16s)

I would like to understand how the usable space is calculated how this affects the Producer Flow Control mechanism and what the best practice to set the storeUsage setting.