[jira] [Commented] (RATIS-1947) TimeoutIOException in WriteLog might not release Pending Requests

["Ivan Andika (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/RATIS-1947?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17813468#comment-17813468 ] 

Ivan Andika commented on RATIS-1947:
------------------------------------

Just an update: we managed to resolve the TimeoutIOException which was caused by the du commands in the datanode volume usage job that's causing a huge inode cache. This huge inode cache causes a lot of direct memory reclaim which blocks the chunk write, causing the TimeoutIOException. We have since migrated to more lightweight DedicatedDiskSpaceUsage to prevent this.

As for the stuck pending request, one observation is that this only happens on Raft peer of a closed pipeline where it has not received the group remove command (similar to the case described in HDDS-9959). Probably because the other Raft followers have been closed and the pending requests cannot be completed since the transaction log entry has never been applied to the leader's state machine. However, I was not able to find the logs to support my hypothesis (e.g. stuck leader election on the unclosed leader) during the stuck pending request so this is just a guess. Maybe someone can help to explain more.

I'm closing this for now, and will reopen this if the problem arises again.

> 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.



--
This message was sent by Atlassian Jira
(v8.20.10#820010)