Snapshot-based downgrades

The snapshot repository format changed in a manner that prevents earlier versions of Elasticsearch from reading the repository contents if it contains snapshots from this version and the last cluster to write to this repository was in the 8.10 series. This will prevent you from reverting an upgrade to the 8.10 series by restoring a snapshot taken before the upgrade.

Snapshot repositories written by clusters running versions 8.11.0 and later are compatible with all earlier versions. Moreover, clusters running version 8.11.0 or later will also automatically repair the repository format the first time they write to the repository to take or delete a snapshot, making it so that all earlier versions can read its contents again.

If you wish to downgrade to a version prior to 8.10.0, take or delete a snapshot using a cluster running version 8.11.0 or later to repair the repository format first. If you cannot repair the repository in this way, first delete all the snapshots in the repository taken with version 8.10.0 or later using a cluster running version 8.10.4.

High Memory Pressure due to a GC change in JDK 21

This version of Elasticsearch is bundled with JDK 21. In JDK 21 Preventive GC has been removed. This may lead to increased memory pressure and an increased number of CircuitBreakerExceptions when retrieving large documents under some particular load. (issue: #99592)

If you needed to explicitly enable Preventive GC for your workload in a previous Elasticsearch version, we recommend you avoid to upgrade to this version, as the settings to enable Preventive GC have been removed from JDK 21.