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From zhangliyun <kelly...@126.com>
Subject Fw:Re:Re: A question about radd bytes size
Date Mon, 02 Dec 2019 21:57:58 GMT












-------- 转发邮件信息 --------
发件人:"zhangliyun" <kellyzly@126.com>
发送日期:2019-12-03 05:56:55
收件人:"Wenchen Fan" <cloud0fan@gmail.com>
主题:Re:Re: A question about radd bytes size



Hi Fan:
   thanks for reply,  I agree that the how the data is stored decides the total bytes of the
table file.
In my experiment,  I found that 
sequence file with gzip compress is 0.5x of the total byte size calculated in memory.
parquet file with lzo compress is 0.2x of the total byte size calculated in memory.


Here the reason why  actual hive table size is  less than total size calculated in memory
is decided by format sequence, orc, parquet and others.
Or is decided by compress algorithm Or both?




Meanwhile can I directly use org.apache.spark.util.SizeEstimator.estimate(RDD) to estimate
the total size of a rdd? I guess there is some difference between the actual size and estimated
size. So in which case, we can use or in which case we can not use.


Best Regards
Kelly Zhang








在 2019-12-02 15:54:19,"Wenchen Fan" <cloud0fan@gmail.com> 写道:

When we talk about bytes size, we need to specify how the data is stored. For example, if
we cache the dataframe, then the bytes size is the number of bytes of the binary format of
the table cache. If we write to hive tables, then the bytes size is the total size of the
data files of the table.


On Mon, Dec 2, 2019 at 1:06 PM zhangliyun <kellyzly@126.com> wrote:

Hi:


 I want to get the total bytes of a DataFrame by following function , but when I insert the
DataFrame into hive , I found the value of the function is different from spark.sql.statistics.totalSize
.  The spark.sql.statistics.totalSize  is less than the result of following function getRDDBytes
. 


   def getRDDBytes(df:DataFrame):Long={

  df.rdd.getNumPartitions match {
case 0 =>
0
case numPartitions =>
val rddOfDataframe = df.rdd.map(_.toString().getBytes("UTF-8").length.toLong)
val size = if (rddOfDataframe.isEmpty()) {
0
} else {
        rddOfDataframe.reduce(_ + _)
      }

      size
  }

}
Appreciate if you can provide your suggestion.


Best Regards
Kelly Zhang






 





 
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