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

["Todd Lipcon (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KUDU-2253?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Todd Lipcon updated KUDU-2253:
------------------------------
    Component/s: perf

> 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: perf, tablet
>    Affects Versions: 1.6.0
>            Reporter: Dan Burkert
>            Assignee: Dan Burkert
>            Priority: Major
>             Fix For: 1.7.0
>
>
> 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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