FAQ
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 127.0.0.1.
* 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 127.0.0.1"
# 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# The next time a 2NN (any 2NN) tries to do a checkpoint, it gets an up-to-date edits file, with an outdated fsimage, and tries to apply those edits to that fsimage.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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