Jira Index
Jira uses a Lucene index stored on the root volume of the Jira cluster nodes. e.g. in...
/var/atlassian/application-data/jira/caches/indexesV1
Troubleshooting
If you experience an outage of a Jira node and when reinstated you appear to be missing Tasks/Issues don't assume data has been lost. The application gets its data from a local Lucene index. The Lucene index acts as a cache for the database. If data is lost from the Lucene index, rebuilding the index will pull the data from the database.
It is recommended to be on a mimum Jira version of 8.19 (JSD/JSM 4.19) to avoid known bugs related to Indexing.
Index Size
Navigate to the JIRA_HOME directory and run the following (for linux)...
du -h /var/atlassian/application-data/jira/caches/indexesV1/
Re-Index
Administration - System - (L) ADVANCED - (L) Indexing