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From Keith Turner <>
Subject Re: How to re-IP an Accumulo Cluster
Date Thu, 15 Aug 2013 15:24:36 GMT
On Thu, Aug 15, 2013 at 11:01 AM, Terry P. <> wrote:

> Greetings everyone,
> We had to re-IP our entire cluster recently to change subnetworks, and we
> essentially lost everything (it was development, so no big deal).  However,
> doing a re-IP operation may be required in actual operational cases, and
> I'd like to know if it can be done or not so we can note it for the future
> (as in document "what not to do" to avoid data loss).
> The issue we had was that after shutting down the cluster, re-IPing all
> servers, and starting everything back up, the tablets were still assigned
> to Tabletservers with the old IP addresses, even though all the hostnames
> were the same.  So the system showed 3 Tabletservers, but no tablets, and
> no entries in the tables where previously there were 400 million.
> So:
> A) Does Zookeeper track Tabletservers by IP address only, and not hostname?

It does track by IP address, but not only IP address.  Each tablet server
has an ephemeral node in zookeeper under the IP address.  This ehpemeral
node should go away when the tserver process dies, and then the master will
assume that tserver is dead.  The location of a tablet in the metadata
table is conceptually <ephemeral node id>+<IP address>, so once that
ephemeral node goes away the location in metadata table is assumed invalid
and the tablet is reassigned.   If another tserver starts at the same IP,
then the master can differentiate because the ephemeral node is different.

You can look at the children nodes under a tserver ip in zookeeper.  Look
at the data for the lowest numbered ephemeral node to to get infor about
who holds the lock for that IP.

> B) If A is true, is there a mechanism to change those entries in Zookeeper
> so that a re-IP operation could be performed?

A first step would be to look in zookeeper and see what going on with the
ephemeral nodes.

In Accumulo 1.3 and 1.4 one thing that normally causes problems when
changing lots of IP addrs is write ahead logs.   Tablets point to their
write ahead logs using the IP address of the logger. This can cause walog
recovery to fail.  In 1.5 walog are stored in HDFS so this not an issue.

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