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  1. Accumulo
  2. ACCUMULO-2668

slow WAL writes

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Details

    • Bug
    • Status: Resolved
    • Blocker
    • Resolution: Fixed
    • None
    • 1.6.0
    • None

    Description

      During continuous ingest, we saw over 70% of our ingest time taken up by writes to the WAL. When we ran the DfsLogger in isolation (created one outside of the Tserver), we saw about ~25MB/s throughput as opposed to nearly 100MB/s from just writing directly to an hdfs outputstream (computed by taking the estimated size of the mutations sent to the DfsLogger class divided by the time it took for it to flush + sync the data to HDFS).

      After investigating, we found one possible culprit was the NoFlushOutputStream. It is a subclass of java.io.FilterOutputStream but does not override the write(byte[], int, int) method signature. The javadoc indicates that subclasses of the FilterOutputStream should provide a more efficient implementation.

      I've attached a small diff that illustrates and addresses the issue but this may not be how we ultimately want to fix it.

      As a side note, I may be misreading the implementation of DfsLogger, but it looks like we always make use of the NoFlushOutputStream, even if encryption isn't enabled. There appears to be a faulty check in the DfsLogger.open() implementation that I don't believe can be satisfied (line 384).

      Attachments

        1. ACCUMULO-2668.0.patch.txt
          1 kB
          Jonathan Park
        2. noflush.diff
          0.9 kB
          Jonathan Park

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              parkjsung Jonathan Park
              parkjsung Jonathan Park
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