Under default settings, which feature of HBase ensures that data won’t be lost in the event of a
RegionServer failure?

A.
All HBase activity is written to the WAL, which is stored in HDFS

B.
All operations are logged on the HMaster.

C.
HBase is ACID compliant, which guarantees that it is Durable.

D.
Data is stored on the local filesystem of the RegionServer.

Explanation:
HBase data updates are stored in a place in memory called memstore for fast write.
In the event of a region server failure, the contents of the memstore are lost because they have
not been saved to disk yet. To prevent data loss in such a scenario, the updates are persisted in a
WAL file before they are stored in the memstore. In the event of a region server failure, the lost
contents in the memstore can be regenerated by replaying the updates (also called edits) from the
WAL file.
Reference: HBase Log Splitting

http://tm.durusau.net/?p=27674(See ‘From the post’ second paragraph)