FAQ
Hi,

I would like to know the relevance of dfs.safemode.extension.
Why would someone wait after leaving the safemode?
Why is it recommended not to set it to 0 instead of 30000 (30 seconds)?

Regards

Bertrand

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  • Harsh J at Sep 21, 2012 at 10:35 am
    For big-large clusters, it helps if the NN waits for N seconds after
    the threshold percentage being satisfied (minimum # of replicas of
    file's blocks being available) so that other DNs get some extra time
    to report in their blocks as well and help ease the initial client
    load the cluster receives. This is where the extension comes useful at
    (certainly tunable to a more suitable value).

    For small clusters (single rack or so) you can probably make it 0 to
    shed off the extra wait.

    However, if you're ever working with NN recovery stuff (one reason the
    NN is down, due to), I recommend setting the threshold itself to >
    1.1f to make sure the NN doesn't auto-exit safemode until you're sure
    that the new inode/block counts are alright and you haven't made any
    mistakes with the recovery process. You can then exit safemode
    manually when sure. In safemode, the NN does not issue block
    deletions, so data loss would not occur out of mistakes made (such as
    starting with an old copy of fsimage accidentally, etc.)
    On Fri, Sep 21, 2012 at 1:47 PM, Bertrand Dechoux wrote:
    Hi,

    I would like to know the relevance of dfs.safemode.extension.
    Why would someone wait after leaving the safemode?
    Why is it recommended not to set it to 0 instead of 30000 (30 seconds)?

    Regards

    Bertrand


    --
    Harsh J

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postedSep 21, '12 at 8:17a
activeSep 21, '12 at 10:35a
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