Contrary to answer by @Harshali Patel, exhaustion is not defined as an uneven distribution, it is rather a cause of it.
A datanode has a property that you can set which defines a threshold of data must be reserved for the OS on that server. Once that limit is exceeded, the datanode process will stop and log an error telling you to delete some files from it. HDFS will continue to function with the other datanodes.
The balancer can be ran to keep storage space healthy and even.