compatibility between hadoop 1.x and hadoop 2.x（hadoop 1.x和2.x的兼容性）
In Apache Hadoop 2.x we have spun off resource management capabilities into Apache Hadoop YARN, a general purpose, distributed application management framework while Apache Hadoop MapReduce (aka MRv2) remains as a pure distributed computation framework.
In general, the previous MapReduce runtime (aka MRv1) has been reused and no major surgery has been conducted on it. Therefore, MRv2 is able to ensure satisfactory compatibility with MRv1 applications. However, due to some improvements and code refactorings, a few APIs have been rendered backward-incompatible.
The remainder of this page will discuss the scope and the level of backward compatibility that we support in Apache Hadoop MapReduce 2.x (MRv2).
在hadoop 2.x我们把资源管理合并到了APACHE HADOOP YARN（分布式管理框架），mapreduce依然是个纯粹的分布式计算框架。
在mapreduce 1.x的基础上开发了mapreduce 2.x，由于一些新实现和代码重构，导致一些API不兼容。
2 binary compatibility：First, we ensure binary compatibility to the applications that use old mapred APIs. This means that applications which were built against MRv1 mapred APIs can run directly on YARN without recompilation, merely by pointing them to an Apache Hadoop 2.x cluster via configuration.
二进制完全兼容
3 source compatibility：We cannot ensure complete binary compatibility with the applications that use mapreduce APIs, as these APIs have evolved a lot since MRv1. However, we ensure source compatibility formapreduce APIs that break binary compatibility. In other words, users should recompile their applications that use mapreduce APIs against MRv2 jars. One notable binary incompatibility break is Counter and CounterGroup.
源代码兼容：用户应该重新编译他们的应用
4 not supported：MRAdmin has been removed in MRv2 because because mradmin commands no longer exist. They have been replaced by the commands in rmadmin. We neither support binary compatibility nor source compatibility for the applications that use this class directly.
mradmin不再使用，被rmadmin取代了。官方意见：既不支持二进制兼容，也不支持源代码兼容，