Because HBase runs on Section 9.9, “HDFS” it is important to understand how it works and how it affects HBase.
The original use-case for HDFS was batch processing. As such, there low-latency reads were historically not a priority. With the increased adoption of Apache HBase this is changing, and several improvements are already in development. See the Umbrella Jira Ticket for HDFS Improvements for HBase.
Since Hadoop 1.0.0 (also 0.22.1, 0.23.1, CDH3u3 and HDP 1.0) via HDFS-2246, it is possible for the DFSClient to take a "short circuit" and read directly from disk instead of going through the DataNode when the data is local. What this means for HBase is that the RegionServers can read directly off their machine's disks instead of having to open a socket to talk to the DataNode, the former being generally much faster[27]. Also see HBase, mail # dev - read short circuit thread for more discussion around short circuit reads.
To enable "short circuit" reads, you must set two configurations.
First, the hdfs-site.xml needs to be amended. Set
the property dfs.block.local-path-access.user
to be the only user that can use the shortcut.
This has to be the user that started HBase. Then in hbase-site.xml,
set dfs.client.read.shortcircuit to be true
For optimal performance when short-circuit reads are enabled, it is recommended that HDFS checksums are disabled.
To maintain data integrity with HDFS checksums disabled, HBase can be configured to write its own checksums into
its datablocks and verify against these. See Section 11.4.9, “hbase.regionserver.checksum.verify”.
The DataNodes need to be restarted in order to pick up the new configuration. Be aware that if a process started under another username than the one configured here also has the shortcircuit enabled, it will get an Exception regarding an unauthorized access but the data will still be read.
A fairly common question on the dist-list is why HBase isn't as performant as HDFS files in a batch context (e.g., as a MapReduce source or sink). The short answer is that HBase is doing a lot more than HDFS (e.g., reading the KeyValues, returning the most current row or specified timestamps, etc.), and as such HBase is 4-5 times slower than HDFS in this processing context. Not that there isn't room for improvement (and this gap will, over time, be reduced), but HDFS will always be faster in this use-case.