Brahma Reddy Battula reopened HDFS-8693:

> refreshNamenodes does not support adding a new standby to a running DN
> ----------------------------------------------------------------------
>                 Key: HDFS-8693
>                 URL: https://issues.apache.org/jira/browse/HDFS-8693
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode, ha
>    Affects Versions: 2.6.0
>            Reporter: Jian Fang
>            Assignee: Ajith S
>            Priority: Critical
>             Fix For: 3.1.0, 2.10.0, 2.9.1, 3.0.1, 2.8.4
>         Attachments: HDFS-8693.02.patch, HDFS-8693.03.patch, HDFS-8693.1.patch
> I tried to run the following command on a Hadoop 2.6.0 cluster with HA 
> support 
> $ hdfs dfsadmin -refreshNamenodes datanode-host:port
> to refresh name nodes on data nodes after I replaced one name node with a new 
> one so that I don't need to restart the data nodes. However, I got the 
> following error:
> refreshNamenodes: 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.
> I checked the 2.6.0 code and the error was thrown by the following code 
> snippet, which led me to this JIRA.
> 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."); }
> }
> Looks like this the refreshNameNodes command is an uncompleted feature. 
> Unfortunately, the new name node on a replacement is critical for auto 
> provisioning a hadoop cluster with HDFS HA support. Without this support, the 
> HA feature could not really be used. I also observed that the new standby 
> name node on the replacement instance could stuck in safe mode because no 
> data nodes check in with it. Even with a rolling restart, it may take quite 
> some time to restart all data nodes if we have a big cluster, for example, 
> with 4000 data nodes, let alone restarting DN is way too intrusive and it is 
> not a preferable operation in production. It also increases the chance for a 
> double failure because the standby name node is not really ready for a 
> failover in the case that the current active name node fails. 

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