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From "Gabriel Reid (JIRA)" <>
Subject [jira] [Commented] (CRUNCH-284) Optimize for minimal disk i/o rather than the number of stages?
Date Mon, 21 Oct 2013 13:57:42 GMT


Gabriel Reid commented on CRUNCH-284:

Yep, definitely a valid use case, although I do think that the onlyOnce in the context of
a write is pretty unintuitive. Another option would be to add another WriteMode.CHECKPOINT
alternative that specifies the read-once semantics, but this would probably be at least equally

I'll try to think of another option, but I do think it's pretty important that we allow reading
checkpointed files with read-once semantics.

> Optimize for minimal disk i/o rather than the number of stages?
> ---------------------------------------------------------------
>                 Key: CRUNCH-284
>                 URL:
>             Project: Crunch
>          Issue Type: Bug
>            Reporter: Chao Shi
> I have a pipeline as follows:
> PCollection in =
> PCollection part1 = f1(in)
> PCollection part2 = f2(in)
> pipelien.write(part1.groupByKey...)
> pipeline.write(part2.groupByKey...)
> where f1 extracts a small potion from "in" and f2 returns the rest. Crunch optimizes
the pipeline into two independent MR jobs, both of which fully read the input.
> I think the ideal MRs should be a map-only job reads the input and split them to two
outputs, and then two MRs read them respectively.
> The problem is that Crunch minimizes the number of MR stages, which is optimal for most
cases, but not optimal in this case. 
> What do you think of this folks?

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