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From "Flavio Junqueira (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3092) Enable journal protocol based editlog streaming for standby namenode
Date Tue, 03 Apr 2012 16:42:26 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3092?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13245475#comment-13245475

Flavio Junqueira commented on HDFS-3092:

Thanks for the responses, Suresh.

bq. For HDFS editlogs, my feeling is that there will only be three JDs. One on the active
namenode, second on the standby and a third JD on one of the machines. In federation, one
has to configure a JD per Federated namespace. Alternative is to use BookKeeper, since it
could make the deployment simpler for federated large cluster.

When you say three JDs, that's the degree of replication, right? When I said multiple logs,
I was referring to multiple namenodes writing to different logs, as with federation.

bq. I think the comment was more about comparing the design and complexity of deployment and
not benchmarks for two systems. Performance is not the motivation for this jira.

Got it. You're thinking about a qualitative design based on the requirements identified. Correctness
sounds like an obvious candidate. :-) 

bq. We decided on majority quorum to keep the design simple, though it is strictly not necessary.
A JD in JournalList is supposed to have all the entries and any JD from the list can be used
to read the journals.

I think my confusion here is that you require a quorum to be able to acknowledge the operation,
but in reality you try to write to everyone. If you can't write to everyone, then you induce
a view change (change to JournalList). Is this right?
> Enable journal protocol based editlog streaming for standby namenode
> --------------------------------------------------------------------
>                 Key: HDFS-3092
>                 URL: https://issues.apache.org/jira/browse/HDFS-3092
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: ha, name-node
>    Affects Versions: 0.24.0, 0.23.3
>            Reporter: Suresh Srinivas
>            Assignee: Suresh Srinivas
>         Attachments: MultipleSharedJournals.pdf
> Currently standby namenode relies on reading shared editlogs to stay current with the
active namenode, for namespace changes. BackupNode used streaming edits from active namenode
for doing the same. This jira is to explore using journal protocol based editlog streams for
the standby namenode. A daemon in standby will get the editlogs from the active and write
it to local edits. To begin with, the existing standby mechanism of reading from a file, will
continue to be used, instead of from shared edits, from the local edits.

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