K8ssandra pod is replaying a large commit log and is not responding - cassandra

Question

We a 3 node Cassandra 4 cluster, at some point (I don't know why) we get in one of the ndoes:
CommitLog.java:173 - Replaying /opt/cassandra/data/commitlog/CommitLog-7-1674673652744.log
With a long list of logs
We can see in the metrics that disk throughput was about 17 GB
During this time we see in the other 2 nodes (the node replaying is not responsive for almost 2m) :
NoSpamLogger.java:98 - /20.9.1.45:7000->prod-k8ssandra-seed-service/20.9.0.242:7000-SMALL_MESSAGES-[no-channel] failed to connect
java.nio.channels.ClosedChannelException: null
at org.apache.cassandra.net.OutboundConnectionInitiator$Handler.channelInactive(OutboundConnectionInitiator.java:248)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:262)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:248)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:241)
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelInactive(DefaultChannelPipeline.java:1405)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:262)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:248)
at io.netty.channel.DefaultChannelPipeline.fireChannelInactive(DefaultChannelPipeline.java:901)
at io.netty.channel.AbstractChannel$AbstractUnsafe$8.run(AbstractChannel.java:819)
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:164)
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:472)
at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:384)
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.base/java.lang.Thread.run(Unknown Source)
Questions:
What is the reason for this commit log replay?
Can we mitigate this node outage risk?
Update:
it seems the restart of node looks like somthing initiated by k8ssandra... this can explain the replay, what is the rason to the HTTP 500? I can't seem to see an
INFO [nioEventLoopGroup-2-2] 2023-01-25 19:07:10,694 Cli.java:617 - address=/127.0.0.6:53027 url=/api/v0/probes/liveness status=200 OK
INFO [nioEventLoopGroup-2-1] 2023-01-25 19:07:12,698 Cli.java:617 - address=http url=/api/v0/probes/readiness status=500 Internal Server Error
INFO [epollEventLoopGroup-38-1] 2023-01-25 19:07:20,700 Clock.java:47 - Using native clock for microsecond precision
WARN [epollEventLoopGroup-38-2] 2023-01-25 19:07:20,701 AbstractBootstrap.java:452 - Unknown channel option 'TCP_NODELAY' for channel '[id: 0x919a5c8b]'
WARN [epollEventLoopGroup-38-2] 2023-01-25 19:07:20,703 Loggers.java:39 - [s33] Error connecting to Node(endPoint=/tmp/cassandra.sock, hostId=null, hashCode=71aac1d0), trying next node (AnnotatedConnectException: connect(..) failed: Connection refused: /tmp/cassandra.sock)
INFO [nioEventLoopGroup-2-2] 2023-01-25 19:07:20,703 Cli.java:617 - address=/127.0.0.6:51773 url=/api/v0/probes/readiness status=500 Internal Server Error
INFO [epollEventLoopGroup-39-1] 2023-01-25 19:07:25,393 Clock.java:47 - Using native clock for microsecond precision
WARN [epollEventLoopGroup-39-2] 2023-01-25 19:07:25,394 AbstractBootstrap.java:452 - Unknown channel option 'TCP_NODELAY' for channel '[id: 0x80b52436]'
WARN [epollEventLoopGroup-39-2] 2023-01-25 19:07:25,395 Loggers.java:39 - [s34] Error connecting to Node(endPoint=/tmp/cassandra.sock, hostId=null, hashCode=cc8ec36), trying next node (AnnotatedConnectException: connect(..) failed: Connection refused: /tmp/cassandra.sock)
INFO [pool-2-thread-1] 2023-01-25 19:07:25,602 LifecycleResources.java:186 - Started Cassandra

Answer 1

When a Cassandra doesn't shutdown cleanly, Cassandra doesn't have a chance to persist the contents of the memtable to disk so when it is restarted, Cassandra replays the commit logs to repopulate the memtables.
It seems like you're confusing cause and effect. The K8ssandra operator restarted the pod because it was unresponsive -- the restart is the effect, not the cause.
You will need to review the Cassandra logs on the pod for clues as to why it became unresponsive. From your description that there was a large commitlog replayed on restart, I would suspect that there was a lot of traffic to the cluster (a large commitlog is a result of lots of writes) and an overloaded node would explain why it became unresponsive. Again, you will need to review the logs to determine the cause.
K8ssandra monitors the pods using "liveness" and "readiness" probes (aka health checks) and the HTTP 500 error would have been a result of the node being unresponsive. This would have triggered the operator to initiate a restart of the pod to automatically recover it. Cheers!