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https://issues.apache.org/jira/browse/HDDS-11955?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17906807#comment-17906807
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Tsz-wo Sze commented on HDDS-11955:
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* raft log replay: a server applies log to state machine
* appendEntries: the leader sends log entries to a follower
A log entry can be applied more than once since a server may (1) applies a log
entry, (2) shuts down before creating a snapshot and (3) restarts.
However, If the leader has successfully sent an entry to the follower, it won't
send the same entry again even if the follower has restarted.
> ContainerStateMachine.readStateMachineData may throw NoSuchFileException
> ------------------------------------------------------------------------
>
> Key: HDDS-11955
> URL: https://issues.apache.org/jira/browse/HDDS-11955
> Project: Apache Ozone
> Issue Type: Bug
> Components: Ozone Datanode
> Reporter: Tsz-wo Sze
> Priority: Major
>
> Suppose we have the following Raft log entires
> - index 110 is a writeChunk,
> - index 120 is a deleteBlock for the chunk above, and
> - one of the datanode followers has nextIndex 100.
> Then, the datanode leader has to send log entries to that follower starting
> from 100. If the leader already has applied log entry 120, it requires to
> read a deleted block in ContainerStateMachine.readStateMachineData and leads
> to NoSuchFileException.
> Also, Ozone datanode does not support Ratis snapshot, this problem cannot be
> worked around by sending a snapshot.
> Potential fix: The datanode leader has to return something instead of
> throwing NoSuchFileException. The datanode leader might return a proto
> indicating that the block is already deleted. When the datanode follower
> receives the proto, it will just mark it as deleted. We need a design for
> this problem.
> Thanks [~sammichen] for pointing out the problem.
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