The health test result for DATA_NODE_BLOCK_COUNT has become concerning: The 
DataNode has 500,008 blocks. Warning threshold: 500,000 block(s).

Having more number of blocks raises the heap requirement at the DataNodes. The
 threshold warning exists to also notify you about this (that you may need to 
soon raise the DN heap size to allow it to continue serving blocks at the same
 performance).


With CM5 we have revised the number to 600k, given memory optimisation 
improvements for DNs in CDH4.6+ and CDH5.0+. You can feel free to raise the
 threshold via the CM -> HDFS -> Configuration -> Monitoring section fields,
 but do look into if your users have begun creating too many tiny files as it 
may hamper their job performance with overheads of too many blocks (and 
thereby, too many mappers).

来源： http://community.cloudera.com/t5/Storage-Random-Access-HDFS/DATA-NODE-BLOCK-COUNT-threshold-200-00-block-s/td-p/12186


‎10-07-2014 10:47 PM

Thanks for your response.
I deleted useless HDFS files(3TB) yesterday(hadoop fs -rm -r), but warning 
messege is still continuous.
DATA_NODE_BLOCK_COUNT is same before deleting files. (current value is 921,891
 blocks)
How can I reduce current DATA_NODE_BLOCK_COUNT?

Even after a file is deleted, the blocks will remain if HDFS Trash is enabled. Do you have Trash enabled? It configured as stated in this URL:
http://www.cloudera.com/documentation/archive/manager/4-x/4-8-6/ClouderaManager-Managing-Clusters/cmmc_hdfs_trash.html