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From Chao Shi <stepi...@live.com>
Subject Re: Question about HBaseSourceTarget#getSize()
Date Sat, 28 Mar 2015 04:27:44 GMT
Hi Nithin,

Because HBaseSourceTarget supports custom Scan criteria (i.e. you can apply
filters), I think it can hardly make a guess on the resulting data size.
Even HBase itself, because of the nature of LSM storage it uses, it cannot
estimate the resulting number of rows or the data size before the query
actually executed.

Does anyone else have better idea on this?

2015-03-17 3:32 GMT+08:00 Nithin Asokan <anithin19@gmail.com>:

> Hello,
> I came across a unique behavior while using HBaseSourceTarget. Suppose I
> have a job(from MRPipeline) that reads from HBase using HBaseSourceTarget
> and passes all the data to a reduce phase, the number of reducers set by
> planner will be equal to 1. The reason being [1]. So, it looks like the
> planner assumes there is only about 1Gb of data that's read from the
> source, and sets the number of reducers accordingly. However, let's say my
> HBase scan is returning very less data or huge amounts of data. The planner
> still assigns 1 reducer(crunch.bytes.per.reduce.task=1Gb). What more
> interesting is, if there are dependent jobs, the planner will set the
> number of reducers based on the initially determined size from HBase source.
>
> As a fix for the above problem, I can set the number of reducers on the
> groupByKey(), but that does not offer much flexibility when dealing with
> data that is of varying sizes. The other option, is to have a map only job
> that reads from HBase and writes to HDFS and have a run(). The next job
> will determine the size right, since FileSourceImpl calculates the size on
> disk.
>
> I noticed the comment on HBaseSourceTarget, and was wondering if there was
> anything planned to have it implemented.
>
> [1]
> https://github.com/apache/crunch/blob/apache-crunch-0.8.4/crunch-hbase/src/main/java/org/apache/crunch/io/hbase/HBaseSourceTarget.java#L173
>
> Thanks
> Nithin
>

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