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https://issues.apache.org/jira/browse/HDFS-5223?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13872771#comment-13872771
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Colin Patrick McCabe commented on HDFS-5223:
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I think the way to do backwards compatible feature flags would be to have a
flag prefix such as "compat_". This would let older NameNodes know that the
(new, unknown to them) flag they were looking at was compatible. The
information needs to be in the flag name, since it can't be in the older
software.
However, I don't think we should implement this now. We don't really have any
infrastructure in place today to use "backwards compatible feature flags."
With the simple DataInputStream/DataOutputStream based decoding we have now,
any extra field results in a loading failure.
So it's fair to say: we can't create even one "backwards compatible feature
flag" without installing new software beyond what this patch provides. Given
that this is true, we should just implement compatible feature flags later when
we know we need (or at least can use) them. I am all for doing it after the
protobuf merge. But there's simply no reason to do it before because I don't
believe a backwards compatible change can be made at this point.
> Allow edit log/fsimage format changes without changing layout version
> ---------------------------------------------------------------------
>
> Key: HDFS-5223
> URL: https://issues.apache.org/jira/browse/HDFS-5223
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: namenode
> Affects Versions: 2.1.1-beta
> Reporter: Aaron T. Myers
> Assignee: Colin Patrick McCabe
> Attachments: HDFS-5223.004.patch
>
>
> Currently all HDFS on-disk formats are version by the single layout version.
> This means that even for changes which might be backward compatible, like the
> addition of a new edit log op code, we must go through the full `namenode
> -upgrade' process which requires coordination with DNs, etc. HDFS should
> support a lighter weight alternative.
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