Your Hadoop cluster has 12 slave nodes, a block size set to 64MB, and a replication factor of
three.
Choose which best describes how the Hadoop Framework distributes block writes into HDFS from
a Reducer outputting a 150MB file?

A.
The Reducer will generate twelve blocks and write them to slave nodes nearest the node on
which the Reducer runs.

B.
The Reducer will generate nine blocks and write them randomly to nodes throughout the
cluster.

C.
The slave node on which the Reducer runs gets the first copy of every block written. Other
block replicas will be placed on other nodes.

D.
Reducers don’t write blocks into HDFS

Explanation:
Note:
*The placement of replicas is critical to HDFS reliability and performance. Optimizing replica
placement distinguishes HDFS from most other distributed file systems. This is a feature that
needs lots of tuning and experience. The purpose of a rack-aware replica placement policy is to
improve data reliability, availability, and network bandwidth utilization. The current implementation
for the replica placement policy is a first effort in this direction. The short-term goals of
implementing this policy are to validate it on production systems, learn more about its behavior,
and build a foundation to test and research more sophisticated policies.
* In HDFS data is split into blocks and distributed across multiple nodes in the cluster. Each block
is typically 64Mb or 128Mb in size. Each block is replicated multiple times. Default is to replicate
each block three times. Replicas are stored on different nodes. HDFS utilizes the local file system
to store each HDFS block as a separate file. HDFS Block size can not be compared with the

traditional file system block size.