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Date: Sat, 9 Mar 2013 18:53:12 +0000 (UTC)
From: "Eric Newton (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (ACCUMULO-1083) add concurrency to HDFS
 write-ahead log
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    [ https://issues.apache.org/jira/browse/ACCUMULO-1083?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13598037#comment-13598037 ] 

Eric Newton commented on ACCUMULO-1083:
---------------------------------------

bq. Does each datanode writing a replica need to complete the write before the TServer will return to the client?

Yes, in the normal case.  However multiple sync's and replications can run in parallel on the same file.
                
> 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, walog-replication-factor-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 continuous ingest 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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