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From "Jakob Homan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1104) Fsck triggers full GC on NameNode
Date Fri, 23 Apr 2010 18:20:51 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1104?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12860341#action_12860341
] 

Jakob Homan commented on HDFS-1104:
-----------------------------------

Would #4 fix the issue we saw completely, while keeping the previously defined behavior that
most people seem to like? It seems that this edge case we've encountered reveals a deficiency
in how we're flushing edits, not in fsck itself.

> Fsck triggers full GC on NameNode
> ---------------------------------
>
>                 Key: HDFS-1104
>                 URL: https://issues.apache.org/jira/browse/HDFS-1104
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: name-node
>            Reporter: Hairong Kuang
>            Assignee: Hairong Kuang
>            Priority: Blocker
>             Fix For: 0.20.3, 0.21.0, 0.22.0
>
>
> A NameNode at one of our clusters fell into full GC while fsck was performed. Digging
into the problem shows that it is caused by how NameNode handles the access time of a file.
> Fsck calls open on every file in the checked directory to get the file's block locations.
Each open changes the file's access time and then leads to writing a transaction entry to
the edit log. The current code optimizes open so that it returns without issuing synchronizing
the edit log to the disk. It happened that in our cluster no other jobs were running while
fsck was performed. No edit log sync was ever called. So all open transactions were kept in
memory. When the edit log buffer got full, it automatically doubled its space by allocating
a new buffer.  Full GC happened when no contiguous space were found when allocating a new
bigger buffer.

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