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From "Ishan Chhabra (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-11558) Caching set on Scan object gets lost when using TableMapReduceUtil in 0.95+
Date Mon, 21 Jul 2014 23:42:40 GMT
Ishan Chhabra created HBASE-11558:

             Summary: Caching set on Scan object gets lost when using TableMapReduceUtil in
                 Key: HBASE-11558
                 URL: https://issues.apache.org/jira/browse/HBASE-11558
             Project: HBase
          Issue Type: Bug
          Components: Scanners
    Affects Versions: 0.95.0
            Reporter: Ishan Chhabra

0.94 and before, if one sets caching on the Scan object in the Job by calling scan.setCaching(int)
and passes it to TableMapReduceUtil, it is correctly read and used by the mappers during a
mapreduce job. This is because Scan.write respects and serializes caching, which is used internally
by TableMapReduceUtil to serialize and transfer the scan object to the mappers.

0.95+, after the move to protobuf, ProtobufUtil.toScan does not respect caching anymore as
ClientProtos.Scan does not have the field caching. Caching is passed via the ScanRequest object
to the server and so is not needed in the Scan object. However, this breaks application code
that relies on the earlier behavior. This will lead to sudden degradation in Scan performance
0.96+ for users relying on the old behavior.

There are 2 options here:
1. Add caching to Scan object, adding an extra int to the payload for the Scan object which
is really not needed in the general case.
2. Document and preach that TableMapReduceUtil.setScannerCaching must be called by the client.

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