[HDDS-8324] Remove chunk cache entry only for the lower index value used for apply transaction

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.4.0
• Component/s: Ozone Datanode
• Labels: pull-request-available
• Assignee: Sumit Agrawal Assign to me
• Reporter: Sumit Agrawal
• Created: 30/Mar/23 06:42
• Updated: 23/Aug/23 17:32
• Resolved: 03/Apr/23 16:01

Description

ContainerStateMachine in DN have stateMachineDataCache, which cache data wrt to logIndex of ratis, this is used by leader to send data to other follower.

Issue: the cache gets cleared with incorrect logic, where all higher index is cleared, when apply transaction is called with lower index.

 

 (division.getInfo().isLeader()) {
  long minIndex = Arrays.stream(division.getInfo()
      .getFollowerNextIndices()).min().getAsLong();
  LOG.debug("Removing data corresponding to log index {} min index {} "
          + "from cache", index, minIndex);
  stateMachineDataCache.removeIf(k -> k >= (Math.min(minIndex, index)));
} 

 

 

Impact: 

• with this clearing, when leader send data, it will cause disk read adding pressure over disk IO.

As solution, the check should be k <= (Math.min(minIndex, index)) where all previous index should be cleared as follower sync is done for that.

 

Impact with this change:

• cache is controlled using LeaderNumPendingRequests (write.element-limit) default 1024 and pendingRequestsBytesLimit (dfs.container.ratis.leader.pending.bytes.limit) default 1GB. So further cache will block till all follower gets sync. This will be correct controlling write load over DN till all cache in sync with majority of follower.

Issue Links

• causes: HDDS-8299 Disk full situation on a leader DN may result in followers getting stuck in a retry loop
• links to: GitHub Pull Request #4499