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From Jing Zhao <j...@hortonworks.com>
Subject Re: Juggling or swaping out the standby NameNode in a QJM / HA configuration
Date Thu, 31 Jul 2014 18:19:49 GMT
Hi Colin,

    I guess currently we may have to restart almost all the
daemons/services in order to swap out a standby NameNode (SBN):

1. The current active NameNode (ANN) needs to know the new SBN since in the
current implementation the SBN tries to send rollEditLog RPC request to ANN
periodically (thus if a NN failover happens later, the original ANN needs
to send this RPC to the correct NN).
2. Looks like the DataNode currently cannot do real refreshment for NN.
Look at the code in BPOfferService:

  void refreshNNList(ArrayList<InetSocketAddress> addrs) throws IOException
{
    Set<InetSocketAddress> oldAddrs = Sets.newHashSet();
    for (BPServiceActor actor : bpServices) {
      oldAddrs.add(actor.getNNSocketAddress());
    }
    Set<InetSocketAddress> newAddrs = Sets.newHashSet(addrs);

    if (!Sets.symmetricDifference(oldAddrs, newAddrs).isEmpty()) {
      // Keep things simple for now -- we can implement this at a later
date.
      throw new IOException(
          "HA does not currently support adding a new standby to a running
DN. " +
          "Please do a rolling restart of DNs to reconfigure the list of
NNs.");
    }
  }

3. If you're using automatic failover, you also need to update the
configuration of the ZKFC on the current ANN machine, since ZKFC will do
gracefully fencing by sending RPC to the other NN.
4. Looks like we do not need to restart JournalNodes for the new SBN but I
have not tried before.

    Thus in general we may still have to restart all the services (except
JNs) and update their configurations. But this may be a rolling restart
process I guess:
1. Shutdown the old SBN, bootstrap the new SBN, and start the new SBN.
2. Keep the ANN and its corresponding ZKFC running, do a rolling restart of
all the DN to update their configurations
3. After restarting all the DN, stop ANN and the ZKFC, and update their
configuration. The new SBN should become active.

    I have not tried the upper steps, thus please let me know if this works
or not. And I think we should also document the correct steps in Apache.
Could you please file an Apache jira?

Thanks,
-Jing



On Thu, Jul 31, 2014 at 9:37 AM, Colin Kincaid Williams <discord@uw.edu>
wrote:

> Hello,
>
> I'm trying to swap out a standby NameNode in a QJM / HA configuration. I
> believe the steps to achieve this would be something similar to:
>
> Use the Bootstrap standby command to prep the replacment standby. Or rsync
> if the command fails.
>
> Somehow update the datanodes, so they push the heartbeat / journal to the
> new standby
>
> Update the xml configuration on all nodes to reflect the replacment
> standby.
>
> Start the replacment standby
>
> Use some hadoop command to refresh the datanodes to the new NameNode
> configuration.
>
> I am not sure how to deal with the Journal switch, or if I am going about
> this the right way. Can anybody give me some suggestions here?
>
>
> Regards,
>
> Colin Williams
>
>

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