[GitHub] [ozone] sodonnel commented on pull request #2395: HDDS-5413. Limit num of containers to process per round for ReplicationManager.

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sodonnel commented on pull request #2395:
URL: https://github.com/apache/ozone/pull/2395#issuecomment-876600258


   > This is just a limit on the number of containers to be processed, note that ReplicationManager count each container as processed no mater it is under replicated or over replicated or good.
   
   I'm not sure how well this will work in practice. Lets say we have 1M containers and the default limit here is 10,000. The default RM sleep interval is 5 minutes. It will take 100 iterations to process all the containers, so 500 minutes. If we lose one node on the cluster, then some percentage of these containers will be under-replicated. Lets say we have a 100 node cluster, so:
   
   1M containers * 3 replicas = 3M / 100 DNs = 30k per DN.
   
   In theory we could process them all in 3 iterations, but this change will not do that.
   
   We also have to consider decommission. In the example I gave above, it might take upto 500 minutes for all the containers on the decommissioning node to get processed. Or worse, for maintenance mode, there may be only a handful of containers that need replicated for the node to goto maintenance, but it will potentially get delayed by 500mins due to the batch size.
   
   RM has problems for sure - I don't like the way it calculates all the replication work for all containers and then sends all the work down to the DNs. The DNs don't really give feedback, the attempts timeout on the RM (by default after 10 minutes) and it schedules more. I feel that we need some way to hold back the work in SCM and feed it to the DNs as they have capacity to accept it. That might mean the DNs feeding back via their heartbeat the pending replication tasks, and then RM not scheduling more commands until the DN has free capacity.
   
   Some DNs on the cluster may work faster than others for some reason (newer hardware, under less load somehow etc). Ideally they would clear their replication work faster than others and hence be able to receive more, but as things stand the work is all passed out randomly, so we cannot take advantage of that.
   
   Another thing we might want to consider, is to trigger RM based on a dead node or decommission / maintenance event, rather than waiting for the thread sleep interval.
   
   I believe we need to have a good think about how RM works, and what we could do to improve it generally.


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