Elasticsearch version 6.3.0

Known issue upgrading from 5.x

Upgrades from any 5.x version will fail for indexes which are prepared using the _synced flush API, or were automatically sync-flushed due to inactivity. If upgrading from those versions, please wait for 6.3.1 to be released.

Clusters with a Gold or Platinum license that are upgrading to 6.3 will need to explicitly set xpack.security.enabled: true in elasticsearch.yml to upgrade successfully. If this value is not set, the cluster will be unable to form after upgrade.

Known issue with Elasticsearch 6.3.x on JDK 10

There appears to be a bug in JDK 10 impacting Elasticsearch 6.3.x (note that the official Docker images are based on JDK 10). The issue appears to impact the C2 compiler on hardware that supports AVX-512 instructions (e.g., Skylake-X CPUs). The issue manifests as a segmentation fault in Java code executing org.apache.lucene.util.MergedIterator#pullTop:

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007fc7ebc050eb, pid=1, tid=151
#
# JRE version: OpenJDK Runtime Environment (10.0.1+10) (build 10.0.1+10)
# Java VM: OpenJDK 64-Bit Server VM (10.0.1+10, mixed mode, tiered, compressed oops, concurrent mark sweep gc, linux-amd64)
# Problematic frame:
# J 14088 c2 org.apache.lucene.util.MergedIterator.pullTop()V (135 bytes) @ 0x00007fc7ebc050eb [0x00007fc7ebc041e0+0x0000000000000f0b]

or executing org.apache.lucene.util.MergedIterator#pushTop:

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007f8c43de5371, pid=1, tid=140
#
# JRE version: OpenJDK Runtime Environment (10.0.1+10) (build 10.0.1+10)
# Java VM: OpenJDK 64-Bit Server VM (10.0.1+10, mixed mode, tiered, compressed oops, concurrent mark sweep gc, linux-amd64)
# Problematic frame:
# J 13039 c2 org.apache.lucene.util.MergedIterator.pushTop()V (92 bytes) @ 0x00007f8c43de5371 [0x00007f8c43de4c40+0x0000000000000731]

A workaround is to disable the C2 compiler from using AVX-512 instructions via the JVM option -XX:UseAVX=2. This issue has been taken upstream to the JDK maintainers and the workaround will be necessary until a version of the JDK that is not prone to this issue is released. For additional background, see #31425.