In Confluence 5.6 the G1 garbage collector was selected as the preferred option.

However, it has come to light that there may be problems with Lucene under this GC algorithm.

See -

• https://issues.apache.org/jira/browse/LUCENE-5168
• https://bugs.openjdk.java.net/browse/JDK-8038348

The Lucene project team themselves report a risk of index corruption, and while Atlassian has seen no evidence of this on our own servers, or heard any customer reports yet, it is prudent to follow the Lucene team's advice and switch our default garbage collector back to the concurrent one.