[jira] [Created] (KUDU-2253) Deltafile on-disk size is 3x larger than expected for large-value workloads

["Dan Burkert (JIRA)" <ji...@apache.org>]

Dan Burkert created KUDU-2253:
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             Summary: Deltafile on-disk size is 3x larger than expected for large-value workloads
                 Key: KUDU-2253
                 URL: https://issues.apache.org/jira/browse/KUDU-2253
             Project: Kudu
          Issue Type: Bug
          Components: tablet
    Affects Versions: 1.6.0
            Reporter: Dan Burkert


While looking into the performance of the integration test written for KUDU-2251 (https://gerrit.cloudera.org/#/c/8951/ revision 6), Todd and I found that the on-disk deltafiles written are about 3x larger than expected.  The culprit is an optimization in the CFile value index which is [turned off for delta files|https://github.com/apache/kudu/blob/branch-1.6.x/src/kudu/tablet/deltafile.cc#L106-L107].  The optimization truncates large keys after the first unique byte between sequential values. The deltafile values, in the case of this integration test, include the small {{DeltaKey}}, and the 8KiB updated value.  As a result the BTree interior nodes are being completely filled by only ~4 values ([32KiB cblock size by default|https://github.com/apache/kudu/blob/branch-1.6.x/src/kudu/tablet/deltafile.cc#L58-L60]).  This makes the BTree far less effective, and means that the full updated data is written many times.  We expect fixing this will improve performance for update-heavy workloads with large values (for example YCSB).



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