Enable disk IO statistics

Disk IO statistics are disabled by default. To enable disk IO statistics, you must set the file IO sampling percentage to a non-zero value in the hdfs-site.xml file.

Set the dfs.datanode.fileio.profiling.sampling.percentage property to a non-zero value in hdfs-site.xml.
```
<property>
  <name>dfs.datanode.fileio.profiling.sampling.fraction</name>
  <value>25</value>
</property>
```
Note
Sampling disk IO might have a minor impact on cluster performance.

	Note
Sampling disk IO might have a minor impact on cluster performance.

Access the disk IO statistics from the NameNode JMX page at http://<namenode_host>:50070/jmx.

In the following JMX output example, the time unit is milliseconds, and the disk is healthy because the IO latencies are low:


    "name" : "Hadoop:service=DataNode,name=DataNodeVolume-/data/disk2/dfs/data/",
    "modelerType" : "DataNodeVolume-/data/disk2/dfs/data/",
    "tag.Context" : "dfs",
    "tag.Hostname" : "n001.hdfs.example.com",
    "TotalMetadataOperations" : 67,
    "MetadataOperationRateAvgTime" : 0.08955223880597014,
  ...
    "WriteIoRateNumOps" : 7321,
    "WriteIoRateAvgTime" : 0.050812730501297636