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The Hadoop Distributed FileSystem Architecture andDesign PDF 下载
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2.4. Simple Coherency Model
HDFS applications need a write-once-read-many access model for files. A file once created,
written, and closed need not be changed. This assumption simplifies data coherency issues
and enables high throughput data access. A Map/Reduce application or a web crawler
application fits perfectly with this model. There is a plan to support appending-writes to files
in the future.
2.5. “Moving Computation is Cheaper than Moving Data”
A computation requested by an application is much more efficient if it is executed near the
data it operates on. This is especially true when the size of the data set is huge. This
minimizes network congestion and increases the overall throughput of the system. The
assumption is that it is often better to migrate the computation closer to where the data is
located rather than moving the data to where the application is running. HDFS provides
interfaces for applications to move themselves closer to where the data is located.
2.6. Portability Across Heterogeneous Hardware and Software Platforms
HDFS has been designed to be easily portable from one platform to another. This facilitates
widespread adoption of HDFS as a platform of choice for a large set of applications.
3. NameNode and DataNodes
HDFS has a master/slave architecture. An HDFS cluster consists of a single NameNode, a
master server that manages the file system namespace and regulates access to files by clients.
In addition, there are a number of DataNodes, usually one per node in the cluster, which
manage storage attached to the nodes that they run on. HDFS exposes a file system
namespace and allows user data to be stored in files. Internally, a file is split into one or more
blocks and these blocks are stored in a set of DataNodes. The NameNode executes file
system namespace operations like opening, closing, and renaming files and directories. It
also determines the mapping of blocks to DataNodes. The DataNodes are responsible for
serving read and write requests from the file system’s clients. The DataNodes also perform
block creation, deletion, and replication upon instruction from the NameNode.
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