Summary

In large data center environment (for example, Jira has over 1 million issues), background reindex was generating load and putting pressure on GC by generating objects and then releasing them after. On top of that, user activities add more load to the system. User might experience performance issue CPU spike and some significant impact.

Documentation

We have mentioned the background reindex impact in Jira Indexing FAQ

Background reindexing uses just a single thread and in theory should trundle along in the background without significant impact on the rest of JIRA. In some circumstances though it may have a significant impact. On large systems under heavy load with a high rate of issue searches then there may be some significant impact, and it may be wise to perform the re-index during a period of lower activity, e.g. in the middle of the night

Suggestion

It would be nice to add some notification or warning message in the index page in JIra's UI indicate the impact for large data center environment.