FAQ
Just trying to understand what happens if there are 3 nodes with
replication set to 3 and one node fails. Does it fail the writes too?

If there is a link that I can look at will be great. I tried searching
but didn't see any definitive answer.

Thanks,
Mohit

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  • Harsh J at Jul 28, 2011 at 7:18 am
    Mohit,

    I believe Tom's book (Hadoop: The Definitive Guide) covers this
    precisely well. Perhaps others too.

    Replication is a best-effort sort of thing. If 2 nodes are all that is
    available, then two replicas are written and one is left to the
    replica monitor service to replicate later as possible (leading to an
    underreplicated write for the moment). The scenario (with default
    configs) would only fail if you have 0 DataNodes 'available' to write
    to.

    Or are you asking about what happens when a DN fails during a write operation?
    On Thu, Jul 28, 2011 at 5:08 AM, Mohit Anchlia wrote:
    Just trying to understand what happens if there are 3 nodes with
    replication set to 3 and one node fails. Does it fail the writes too?

    If there is a link that I can look at will be great. I tried searching
    but didn't see any definitive answer.

    Thanks,
    Mohit


    --
    Harsh J
  • Mohit Anchlia at Jul 28, 2011 at 4:19 pm

    On Thu, Jul 28, 2011 at 12:17 AM, Harsh J wrote:
    Mohit,

    I believe Tom's book (Hadoop: The Definitive Guide) covers this
    precisely well. Perhaps others too.

    Replication is a best-effort sort of thing. If 2 nodes are all that is
    available, then two replicas are written and one is left to the
    replica monitor service to replicate later as possible (leading to an
    underreplicated write for the moment). The scenario (with default
    configs) would only fail if you have 0 DataNodes 'available' to write
    to.
    Thanks Harsh. I think you answered my question. I thought that
    replication of 3 is a must. And for that you really need atleast 4
    nodes so that if one of the nodes die it can still write to 3 nodes. I
    am assuming writes to replica nodes are always synchronous and not
    eventually consistent.
    Or are you asking about what happens when a DN fails during a write operation?
    I am assuming there will be some errors in this case.
    On Thu, Jul 28, 2011 at 5:08 AM, Mohit Anchlia wrote:
    Just trying to understand what happens if there are 3 nodes with
    replication set to 3 and one node fails. Does it fail the writes too?

    If there is a link that I can look at will be great. I tried searching
    but didn't see any definitive answer.

    Thanks,
    Mohit


    --
    Harsh J

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postedJul 27, '11 at 11:39p
activeJul 28, '11 at 4:19p
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