When in 'yarn-client' mode, the spark charm creates a directory in hdfs with 'spark' user/group permissions.
If spark is not colocated with the NN and RM, this user/group does not exist on those machines. This causes warnings in the hdfs log like this:
WARN org.apache.hadoop.security.UserGroupInformation: No groups available for user spark
It also prevents the RM history server from accessing spark job history files.
To combat this, always include a spark user/group in the NN/RM charms. This is very similar to how we always create a 'mapred' user on the NN to prevent warnings about mapred being unknown to the namenode.