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From Lance Norskog <goks...@gmail.com>
Subject Re: How do I diagnose IO bounded errors using the framework counters?
Date Thu, 29 Sep 2011 23:29:01 GMT
When in doubt, go straight to the owner of a fact. The operating system is
what really knows disk i/o.
"my mapper job--which may write multiple <key,value> pairs for each one it
receives--is writing too many" - ah, a map-increase job :) This is what
Combiners are for- to keep explosions of data from hitting the network by
combining in the mapper machine.

On Thu, Sep 29, 2011 at 4:15 PM, W.P. McNeill <billmcn@gmail.com> wrote:

> I have a problem where certain Hadoop jobs take prohibitively long to run.
> My hypothesis is that I am generating more I/O than my cluster can handle
> and I need to substantiate this. I am looking closely at the Map Reduce
> framework counters because I think they contain the information I need, but
> I don't understand what the various File System Counters are telling me. Is
> there a pointer to an list of exactly what all these counters mean? (So far
> my online research has only turned up other people asking the same
> question.)
> In particular, I suspect that my mapper job--which may write multiple <key,
> value> pairs for each one it receives--is writing too many and the values
> are too large, but I'm not sure how to test this quantitatively.
> Specific questions:
>   1. I assume "Map input records" is the total of all <key, value> pairs
>   coming into the mappers and "Map output records" is the total of all
> <key,
>   value> pairs written by the mapper. Is this correct?
>   2. What is "Map output bytes"? Is this the total number of bytes in all
>   the <key, value> pairs written by the mapper?
>   3. How would I calculate a corresponding "Map input bytes"? Why doesn't
>   that counter exist?
>   4. What is the relationship between the FILE|HDFS_BYTES_READ|WRITTEN
>   counters? What exactly do they mean, and how do they relate to the "Map
>   output bytes" counter?
>   5. Sometimes the FILE bytes read and written values are an order of
>   magnitude larger than the corresponding HDFS values, and sometimes it's
> the
>   other way around. How do I go about interpreting this?

Lance Norskog

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