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Date: Tue, 28 Feb 2012 23:52:54 +0000 (UTC)
From: "Todd Lipcon (Updated) (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Updated] (HDFS-3025) Automatic log sync shouldn't happen
 inside logEdit path
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     [ https://issues.apache.org/jira/browse/HDFS-3025?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Todd Lipcon updated HDFS-3025:
------------------------------

    Attachment: hdfs-3025.txt

Fairly simple patch.

I tested this out on a 100-node cluster running the HA branch with HA off -- a workload which causes a lot of non-synced log edits. I could see the logsyncs happening on the correct thread using jstack, and verified that performance was improved, with fewer threads blocked.
                
> Automatic log sync shouldn't happen inside logEdit path
> -------------------------------------------------------
>
>                 Key: HDFS-3025
>                 URL: https://issues.apache.org/jira/browse/HDFS-3025
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node, performance
>    Affects Versions: 0.23.3
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hdfs-3025.txt
>
>
> HDFS-3020 fixes the "automatic log sync" functionality so that, when logEdits is called without log sync, it eventually triggers a sync. That sync ends up being inline, though, which means the FSN lock is usually held during it. This causes a bunch of threads to pile up.
> Instead, we should have it just set a "syncNeeded" flag and trigger a sync from another thread which isn't holding the lock (or from the same thread using a "logSyncIfNeeded" call).
> (credit to the FB branch for this idea)

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