從官網來copy過來的幾種模式描述：
| 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就是出現錯誤后，拋出錯誤
Append顧名思義，就是追加信息
Overwrite是覆蓋
Ignore是如果存在則忽略
另外，如果沒有指定存儲模式，那么默認應該是SaveMode.ErrorIfExists，因為我重復保存后報了：already exists錯誤來着。
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換成其他的幾種形式嘍