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From "Aaron T. Myers (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-2305) Running multiple 2NNs can result in corrupt file system
Date Wed, 31 Aug 2011 21:06:10 GMT
Running multiple 2NNs can result in corrupt file system

                 Key: HDFS-2305
                 URL: https://issues.apache.org/jira/browse/HDFS-2305
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: name-node
    Affects Versions: 0.20.2
            Reporter: Aaron T. Myers
            Assignee: Aaron T. Myers

Here's the scenario:

* You run the NN and 2NN (2NN A) on the same machine.
* You don't have the address of the 2NN configured, so it's defaulting to
* There's another 2NN (2NN B) running on a second machine.
* When a 2NN is done checkpointing, it says "hey NN, I have an updated fsimage for you. You
can download it from this URL, which includes my IP address, which is x"

And here's the steps that occur to cause this issue:

# Some edits happen.
# 2NN A (on the NN machine) does a checkpoint. All is dandy.
# Some more edits happen.
# 2NN B (on a different machine) does a checkpoint. It tells the NN "grab the newly-merged
fsimage file from"
# NN happily grabs the fsimage from 2NN A (the 2NN on the NN machine), which is stale.
# NN renames edits.new file to edits. At this point the in-memory FS state is fine, but the
on-disk state is missing edits.
# The next time a 2NN (any 2NN) tries to do a checkpoint, it gets an up-to-date edits file,
with an outdated fsimage, and tries to apply those edits to that fsimage.
# Kaboom.

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