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https://issues.apache.org/jira/browse/RATIS-1947?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17795673#comment-17795673
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Ivan Andika commented on RATIS-1947:
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Hi [~szetszwo] , currently I'm still unable to pinpoint the root cause on why
the pending request are stuck. However it does not happen as often in our
cluster anymore.
Our Ozone cluster uses internal Ratis version so please go ahead with the Ratis
3.0.1 bug fix. If there is any solution for this issue, I can put this under
the subsequent Ratis release.
Thank you for the help. I will continue to address this when I have more time.
> TimeoutIOException in WriteLog might not release Pending Requests
> -----------------------------------------------------------------
>
> Key: RATIS-1947
> URL: https://issues.apache.org/jira/browse/RATIS-1947
> Project: Ratis
> Issue Type: Bug
> Components: Leader
> Reporter: Ivan Andika
> Assignee: Ivan Andika
> Priority: Major
> Attachments: image-2023-12-01-12-04-24-409.png,
> image-2023-12-01-13-28-24-125.png
>
>
> Recently there are some I/O issues in our Ozone cluster's datanodes that
> causes WriteLog to time out with this exceptions
> {code:java}
> 2023-11-27 18:05:36,593
> [dac3eec1-7886-4278-b0db-d6b62a046d44@group-2BEC613EA29B-SegmentedRaftLogWorker]
> WARN org.apache.ratis.server.raftlog.segmented.SegmentedRaftLogWorker:
> Timeout 0/~
> org.apache.ratis.protocol.exceptions.TimeoutIOException: Timeout 10s:
> WriteLog:337065866: (t:7, i:337065866), STATEMACHINELOGENTRY, cmdType:
> WriteChunk traceID: "" containerID: 3470188 datanodeUuid:
> "18b90e19-6a7d-4858-a907-4acdf04a3949" pipelineID:
> "9de85e00-968d-4b32-a589-2bec613ea29b" writeChunk { blockID { containerID:
> 3470188 localID: 107544269209449942 blockCommitSequenceId: 337065456 }
> chunkData { chunkName: "107544269209449942_chunk_9" offset: 33554432 len:
> 4194304 checksumData { type: CRC32 bytesPerChecksum: 1048576 checksums:
> "\224\316\216\327" checksums: "\326\223/\201" checksums: "O\3374\336"
> checksums: "\313\231Bf" } } }, container
> path=/mnt/disk/3/ozone/hdds/CID-354bf147-15b8-4d31-9ff0-c0bce64451cf/current/containerDir121/3470188-writeStateMachineData
> at org.apache.ratis.util.IOUtils.getFromFuture(IOUtils.java:89)
> at
> org.apache.ratis.server.raftlog.segmented.SegmentedRaftLogWorker$StateMachineDataPolicy.getFromFuture(SegmentedRaftLogWorker.java:89)
> at
> org.apache.ratis.server.raftlog.segmented.SegmentedRaftLogWorker$WriteLog.execute(SegmentedRaftLogWorker.java:561)
> at
> org.apache.ratis.server.raftlog.segmented.SegmentedRaftLogWorker.run(SegmentedRaftLogWorker.java:321)
> at java.lang.Thread.run(Thread.java:748)
> Caused by: java.util.concurrent.TimeoutException
> at
> java.util.concurrent.CompletableFuture.timedGet(CompletableFuture.java:1784)
> at
> java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1928)
> at org.apache.ratis.util.IOUtils.getFromFuture(IOUtils.java:80)
> ... 4 more {code}
> This translate to state machine data write timeout shown in the metrics
> dashboard
> !image-2023-12-01-13-28-24-125.png!
> After these exceptions, it's seen that the Pending Requests in leader queue
> is stuck and unable to return to normal, even after the I/O performance
> returns to normal
> !image-2023-12-01-12-04-24-409.png!
> While the leader pending request limit hits did not increase, which means
> that no client requests failed with ResourceUnavailableException (but there
> were previous instances when these limits are hit). It seems that the client
> operations are blocked.
> My expectations are that once I/O performance has returned, leader's pending
> requests should have been removed, which also release the RequestLimits
> semaphore. Looking at the LeaderStateImpl there are some pending
> requests-related methods:
> * Acquiring pending requests: called in RaftServerImpl#appendTransaction
> after StateMachine (e.g. ContainerStateMachine) finish building the
> TransactionContext in StateMachine#startTransaction
> ** tryAcquirePendingRequest: Acquiring the permit based on the current size
> and number of pending requests
> *** If the pending requests size and limits are hit,
> ResourceUnavailableException should be thrown
> ** addPendingRequest: Using the permit, add the client request to the
> pending requests
> * Releasing pending requests: called when applying the log to state machine
> (means the transaction / log entry has been successfully committed)
> ** replyPendingRequest: Will release pending request element and size
> Looking at the codes for a bit, I suspected that TimeoutIOException of
> WriteLog did not release the leader's pending requests which seems to cause
> some client operations to be blocked. However, I'm not entirely sure the
> expected behavior when TimeoutIOException is thrown (will it be retried
> indefinitely?).
> Our Ozone version is still using the Ratis version based on 2.4.1 release.
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