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https://issues.apache.org/jira/browse/ACCUMULO-1083?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13596488#comment-13596488
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Keith Turner commented on ACCUMULO-1083:
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bq. We suspect that the performance hit has to do with the Hadoop bottleneck
described above.
Look into replication. 1.4 may default to 2 replicas and 1.5 may default to 3
replicas for walogs.
> add concurrency to HDFS write-ahead log
> ---------------------------------------
>
> Key: ACCUMULO-1083
> URL: https://issues.apache.org/jira/browse/ACCUMULO-1083
> Project: Accumulo
> Issue Type: Improvement
> Components: tserver
> Reporter: Adam Fuchs
> Fix For: 1.6.0
>
> Attachments: walog-performance.jpg
>
>
> When running tablet servers on beefy nodes (lots of disks), the write-ahead
> log can be a serious bottleneck. Today we ran a test of 1.5-SNAPSHOT on an
> 8-node (plus a master node) cluster in which the nodes had 32 cores and 15
> drives each. Running with write-ahead log off resulted in a >4x performance
> improvement sustained over a long period.
> I believe the culprit is that the WAL is only using one file at a time per
> tablet server, which means HDFS is only appending to one drive (plus
> replicas). If we increase the number of concurrent WAL files supported on a
> tablet server we could probably drastically improve the performance on
> systems with many disks. As it stands, I believe Accumulo is significantly
> more optimized for a larger number of smaller nodes (3-4 drives).
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