Todd Lipcon created HDFS-4025:
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             Summary: QJM: Sychronize past log segments to JNs that missed them
                 Key: HDFS-4025
                 URL: https://issues.apache.org/jira/browse/HDFS-4025
             Project: Hadoop HDFS
          Issue Type: Sub-task
          Components: ha
    Affects Versions: QuorumJournalManager (HDFS-3077)
            Reporter: Todd Lipcon
            Assignee: Todd Lipcon


Currently, if a JournalManager crashes and misses some segment of logs, and 
then comes back, it will be re-added as a valid part of the quorum on the next 
log roll. However, it will not have a complete history of log segments (i.e any 
individual JN may have gaps in its transaction history). This mirrors the 
behavior of the NameNode when there are multiple local directories specified.

However, it would be better if a background thread noticed these gaps and 
"filled them in" by grabbing the segments from other JournalNodes. This 
increases the resilience of the system when JournalNodes get reformatted or 
otherwise lose their local disk.

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