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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-970) FSImage writing should always fsync before close
Date Thu, 11 Feb 2010 23:42:27 GMT

    [ https://issues.apache.org/jira/browse/HDFS-970?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12832738#action_12832738
] 

Todd Lipcon commented on HDFS-970:
----------------------------------

Relevant LKML thread:
http://lkml.indiana.edu/hypermail/linux/kernel/0903.3/01327.html
{quote}
Note that the rename for atomic commits trick originated in mail severs
which always did the proper fsync. When the word spread into the
desktop world it looks like this wisdom got lost.
{quote}

> FSImage writing should always fsync before close
> ------------------------------------------------
>
>                 Key: HDFS-970
>                 URL: https://issues.apache.org/jira/browse/HDFS-970
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: name-node
>    Affects Versions: 0.20.1, 0.21.0, 0.22.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>            Priority: Critical
>
> Without an fsync, it's common that filesystems will delay the writing of metadata to
the journal until all of the data blocks have been flushed. If the system crashes while the
dirty pages haven't been flushed, the file is left in an indeterminate state. In some FSs
(eg ext4) this will result in a 0-length file. In others (eg XFS) it will result in the correct
length but any number of data blocks getting zeroed. Calling FileChannel.force before closing
the FSImage prevents this issue.

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