[jira] [Created] (ACCUMULO-2915) Avoid copying all Mutations when using a TabletServerBatchWriter

["William Slacum (JIRA)" <ji...@apache.org>]

William Slacum created ACCUMULO-2915:
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             Summary: Avoid copying all Mutations when using a TabletServerBatchWriter
                 Key: ACCUMULO-2915
                 URL: https://issues.apache.org/jira/browse/ACCUMULO-2915
             Project: Accumulo
          Issue Type: Improvement
          Components: client
    Affects Versions: 1.6.0, 1.5.1, 1.5.0, 1.6.1, 1.7.0
            Reporter: William Slacum
             Fix For: 1.5.2, 1.6.1, 1.7.0


Currently in the TabletServerBatchWriter, the following behavior is exhibited:

{code}
    // create a copy of mutation so that after this method returns the user
    // is free to reuse the mutation object, like calling readFields... this
    // is important for the case where a mutation is passed from map to reduce
    // to batch writer... the map reduce code will keep passing the same mutation
    // object into the reduce method
    m = new Mutation(m);
    
    totalMemUsed += m.estimatedMemoryUsed();
    mutations.addMutation(table, m);
    totalAdded++;
{code}

This means all data is copied twice when writing. The logic for doing this is a bit dubious, since not all clients are going to be subject to MapReduce's use of references. 

It'd be good if we provided users with a way of signaling that there's no need to copy the mutation payload. [~elserj] suggested creating something akin to an {{ImmutableMutation}}, which help avoid some of the fears the batchwriter attempts to defend against.



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