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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2305) Running multiple 2NNs can result in corrupt file system
Date Mon, 05 Sep 2011 18:58:10 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2305?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13097249#comment-13097249
] 

Todd Lipcon commented on HDFS-2305:
-----------------------------------

- Some of the new info messages should probably be debug level
- Do we also need to add some locking so that only one 2NN could be uploading an image at
the same time? eg what about the following interleaving:
-- 2NN A starts uploading a good checkpoint which is large
-- 2NN B starts uploading an invalid checkpoint which is small, which overwrites fsimage.ckpt
-- 2NN B gets the cksum error, leaving its bad fsimage.ckpt in place
-- 2NN A finishes uploading, and calls checkpointUploadDone - B's fsimage.ckpt is rolled into
place
- getNewChecksum looks like it will leak a file descriptor - need a try/finally close
- would it be easier to just backport the part of 903 that creates an "imageChecksum" member
which is updated whenever the image is merged, by the existing output stream? That would reduce
divergence between 20s and trunk. That is to say, backport HDFS-903 except for the part where
the checksum is put in the VERSION file.

> 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
>         Attachments: hdfs-2305-test.patch, hdfs-2305.0.patch
>
>
> 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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