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For patch releases (x.y.z), run the release wizard script from the release branch, e.g. branch_8_8; for minor release, run the release wizard from the major version branch, e.g. branch_8x; for major release, run the release wizard from master.
The release wizard prompts whether you want to do the release in a new, clean checkout under ~/.lucene-releases. Absolutely do this and don't try to save disk or network I/O by using your normal working dir for the project!
To speed up tests while building the RC, you can increase parallelism by adding the following settings to your ~/lucene.build.properties
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