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From Krishmin Rai <>
Subject Re-init Accumulo over existing installation
Date Thu, 05 Jul 2012 15:15:23 GMT
Hi All,
  We've recently encountered a strange situation on a small test cluster: after an awkward
crash, our ZooKeeper data was erased and we no longer have the [accumulo] znode. The HDFS
accumulo directory is intact, so all the RFiles and etc are still there, but it's not clear
how best to bring Accumulo back up to its previous state. Obviously just starting Accumulo
as-is complains about the missing znode ("Waiting for accumulo to be initialized"), whereas
re-initializing is not possible over existing HDFS directories ("It appears this location
was previously initialized, exiting").

  A couple of questions about recovery strategies:

1) Is there any way to re-create the znode for a previous instance-id? My understanding is
that ZK is mostly used to store ephemeral data (such as which tserver is currently responsible
for which tablets) and things like users (which we could re-create), so perhaps this is plausible?

2) I imagine that I could init Accumulo with a new instance.dfs.dir, then import the RFiles
from the old installation back in. I see Patrick just asked a related question, so, with the
data integrity caveats, I would essentially be following the last of the steps in ACCUMULO-456.

3) This is a vague question, but have any of you had experience with the [accumulo] znode
being entirely deleted? Aside from stopping/starting ZK (3.3.5) and Accumulo 1.4.0 (possibly
with a force-quit), I'm not sure what we could have done to actually delete it.

This is just a test instance, and the data could easily be recreated, but I want to take this
opportunity to learn a little more about Accumulo plumbing and maintenance.



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