HDFS now allows for disks to fail in place, without affecting DataNode operations, until a threshold value is reached. This is set on each DataNode via the dfs.datanode.failed.volumes.tolerated property; it defaults to 0, meaning that any volume failure will shut down the DataNode; on a production cluster where DataNodes typically have 6, 8, or 12 disks, setting this parameter to 1 or 2 is typically the best practice.