Replicate container commands
Learn about replicate container command, types of this command, and the configuration that you can use to control it, and
Ozone's Replication Manager (RM) sends this command to a datanode that contains a replica of the container, instructing the datanode to replicate this to another target datanode. The replicate container command is sent to the datanode with the least commands queued.
If all sources have too many commands queued, the container cannot be replicated, and the command is re-queued to be tried again later.
There are two types of replication commands; simple replication and EC reconstruction. The simple replication and EC reconstruction commands share the same datanode queue and worker thread pool on the datanode, and hence they have a single combined limit. As EC reconstruction commands are more expensive to process than simple replication commands, the EC reconstruction commands are given a weighting so that queuing one command counts 1 * weight to the limit. The default weight is currently 3. For more information, see EC reconstruction commands.
configuration to adjust the limit of the number of simple replication and EC reconstruction
commands that can be queued on a datanode.
The balancer and low priority commands
The Container Balancer service also sends replicate commands through the RM API to balance the utilization of datanodes in an Ozone cluster.
The Container Balancer can create a large number of replicate container commands on
the datanode, so that the Container Balancer does not impact the more important work performed
by RM. The Replicate Container commands can be sent with two priorities; normal and low. The
Container Balancer always sends low priority replicate container commands, while the RM always
sends normal priority commands. Low priority commands do not count towards the command limit
configured by the
configuration. If the datanode has normal priority commands queued, the low priority commands
are not processed. That way, if there is a large amount of Balancer work scheduled, and some
essential replication work is required, replication work gets priority.
The Container Balancer also schedules commands with a larger timeout, to give time for its work to complete and also to cater for any higher priority commands which might slow its progress.