从官网来copy过来的几种模式描述：
| Scala/Java | Python | Meaning |
|---|---|---|
SaveMode.ErrorIfExists(default) | "error"(default) | When saving a DataFrame to a data source, if data already exists, an exception is expected to be thrown. |
SaveMode.Append | "append" | When saving a DataFrame to a data source, if data/table already exists, contents of the DataFrame are expected to be appended to existing data. |
SaveMode.Overwrite | "overwrite" | Overwrite mode means that when saving a DataFrame to a data source, if data/table already exists, existing data is expected to be overwritten by the contents of the DataFrame. |
SaveMode.Ignore | "ignore" | Ignore mode means that when saving a DataFrame to a data source, if data already exists, the save operation is expected to not save the contents of the DataFrame and to not change the existing data. This is similar to a `CREATE TABLE IF NOT EXISTS` in SQL. |
ErrorIfExists就是出现错误后，抛出错误
Append顾名思义，就是追加信息
Overwrite是覆盖
Ignore是如果存在则忽略
另外，如果没有指定存储模式，那么默认应该是SaveMode.ErrorIfExists，因为我重复保存后报了：already exists错误来着。
如何使用：
import org.apache.spark.sql._
val sqlContext = new org.apache.spark.sql.SQLContext(sc)
val df = sqlContext.load("/opt/modules/spark1.3.1/examples/src/main/resources/people.json")
df.save("/opt/test/1","json", SaveMode.Overwrite) //可以把SaveMode.Overwrite换成其他的几种形式喽
