Gaurav Narula created KAFKA-19474:
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Summary: Wrong placement of WARN log for truncation below HWM
Key: KAFKA-19474
URL: https://issues.apache.org/jira/browse/KAFKA-19474
Project: Kafka
Issue Type: Bug
Components: core
Affects Versions: 4.0.0, 4.2.0
Reporter: Gaurav Narula
{{ReplicaFetcherThread#truncate}} has a useful [WARN
log|https://github.com/apache/kafka/blob/da4fbba2793528e283458e080a690ad141857b0b/core/src/main/scala/kafka/server/ReplicaFetcherThread.scala#L171]
when a follower is truncating its log segment below its HWM.
This is of particular help when debugging data loss scenarios like those
described in the motivation for
[KIP-966|https://cwiki.apache.org/confluence/display/KAFKA/KIP-966%3A+Eligible+Leader+Replicas].
Unfortunately, a refactoring in [PR
5608|https://github.com/apache/kafka/pull/5608/files#diff-693a5eaaaa4fe61a18af23d5b06ccc024261dda01558cbaacb7020e1911c6d55R264]
moved the check to _after_ the truncation, which may modify the HWM. As a
result, the log may not be emitted even though the log is truncated below the
HWM
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