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From "Samer Al-Kiswany (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-6820) Namenode fails to boot if the file system reorders rename operations
Date Tue, 05 Aug 2014 17:36:13 GMT
Samer Al-Kiswany created HDFS-6820:
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             Summary: Namenode fails to boot if the file system reorders rename operations
                 Key: HDFS-6820
                 URL: https://issues.apache.org/jira/browse/HDFS-6820
             Project: Hadoop HDFS
          Issue Type: Bug
    Affects Versions: 2.2.0
            Reporter: Samer Al-Kiswany
            Priority: Minor


After studying the steps HDFS name node takes to update the logs we found the following bug.
The bug may not manifest in all current file system implementations, but it is possible in
file systems that reorder metadata operations. e.g. btrfs

Looking at the strace of HDFS name node we see the following when updating the image:
create(fsimage.chk)
append(fsimage.chk)
fsync(fsimage.chk)

create(fsimage.md5.tmp)
append(fsimage.md5.tmp)
fsync(fsimage.md5.tmp)

rename(fsimage.md5, fsimage.md5.tmp)
rename(fsimage, fsimage.chk)

If the file system reorders the last two rename operations and the system crashes before the
second rename is persisted on disk, the system may end up with a fsimage that does not have
a corresponding md5 file. In this case HDFS namenode does not boot.




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