Upgrading from 0.18 requires issuing a full flush of all the indices in the cluster (curl host:9200/_flush) before shutting down the cluster, with no indexing operations happening after the flush.
The local gateway state structure has changed, a backup of the state files is created when upgrading, they can then be used to downgrade back to 0.18. Don’t downgrade without using them.
New features:
Index Blocks: Add index.blocks.write, index.blocks.read, and index.blocks.metadata settings (#1771)
Enhancements:
Add cache thread pool to handle cache loading of async caches (bloom filter) (#1777)