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From Michael Segel <michael_se...@hotmail.com>
Subject Re: Occasional regionserver crashes following socket errors writing to HDFS
Date Fri, 11 May 2012 02:46:33 GMT

Since you brought it up...
> http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/mapreduce/package-summary.html#sink.

"Writing, it may make sense to avoid the reduce step and write yourself back into HBase from
inside your map. You'd do this when your job does not need the sort and collation that mapreduce
does on the map emitted data; on insert, HBase 'sorts' so there is no point double-sorting
(and shuffling data around your mapreduce cluster) unless you need to. If you do not need
the reduce, you might just have your map emit counts of records processed just so the framework's
report at the end of your job has meaning or set the number of reduces to zero and use TableOutputFormat.
See example code below. If running the reduce step makes sense in your case, its usually better
to have lots of reducers so load is spread across the HBase cluster."

This isn't 100% true. 

I'd lose the quotes around 'sorts' because the data is sorted on key values. period.

I'd ask that you reconsider the following phrase...
"You'd do this when your job does not need the sort and collation that mapreduce does on the
map emitted data;" 

I realize I went to this little midwestern school (tOSU), where ENG meant you were in the
college of engineering and not an English Major, so I'm not sure if I am parsing that statement

If you refactor your M/R , HBase can be used for the 'collation' .  (If you make your Mapper
a null writable and manually write the output to HBase within Mapper.map(), you can write
to N tables without a problem. So you can write the record out, update a table where you are
keeping counters, stats, etc ... )  So I am still at a loss to find an example of where you
would need a reducer. 

Don't get me wrong. I do believe that there are cases where you may need a reducer, just as
I believe that there is intelligent life on other planets. I just haven't found it yet. Of
course YMMV.

Which is why I ask you to think really long and hard on this issue. 

With respect to Eran's problem... 

He's writing sorted output to Hbase. 
He stated that this problem happens with heavy writes. 
And that its worse when he has more reducers. (Something recommended in the paragraph...)

So one has to ask what would cause a write to be blocked 
GC ? Eran says he's already tuned it.
MSLABS? Eran says that's covered. 

Table splits? 
Eran says that the table's region sizes are 256MB (default) and the other table is 512MB.

If the table is constantly splitting, then you need to increase the region size. Again we
don't have enough information to diagnose if this is the issue.

We don't know things about his cluster like the number of nodes, how much memory on each node,
as well as which version of HBase.

I realize that these are all pretty basic issues, but sometimes its the little things that
will trip you up. 



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