Why the flow-control reports "Persistent store is Full" even I increase the disk during A-MQ running?

Issue

• When A-MQ is running, I see the storage which used by KahaDB has no more space left, so I increased it to 20GB immediately.
  However several minutes later, flow-control is triggered to report disk shortage:

2018-01-17 09:56:13,214 | INFO  | 25.3:44852@61616 | Queue                            | mq.broker.region.BaseDestination  683 | 162 - org.apache.activemq.activemq-osgi - 5.11.0.redhat-630283 | Usage(default:store:queue://a.b.c.1:store) percentUsage=99%, usage=32051226594, limit=32050362970, percentUsageMinDelta=1%;Parent:Usage(default:store) percentUsage=100%, usage=32051226594, limit=32050362970, percentUsageMinDelta=1%: Persistent store is Full, 100% of 32050362970. Stopping producer (ID:SNTP1IMT01-33661-1516100975712-1:15:1:1) to prevent flooding queue://a.b.c.1. See http://activemq.apache.org/producer-flow-control.html for more info (blocking for: 31s)