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https://issues.apache.org/jira/browse/HDFS-5223?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14483704#comment-14483704
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Sanjay Radia commented on HDFS-5223:
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The above solution was inspired by Hive's ORC. They have two complementary
mechanisms to address dealing with old and new binaries. They specify the
oldest version that can safely read the new data (which inspired the solution i
gave above) and also new binaries can write in older format. This second
mechanim is too burdensome for HDFS. Instead I would prefer to disable the new
new features after which one cannot downgrade.
> 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.
> Copied description from HDFS-8075 which is a duplicate and now closed.
> Background
> * HDFS image layout was changed to use Protobufs to allow easier forward and
> backward compatibility.
> * Hdfs has a layout version which is changed on each change (even if it an
> optional protobuf field was added).
> * Hadoop supports two ways of going back during an upgrade:
> ** downgrade: go back to old binary version but use existing image/edits so
> that newly created files are not lost
> ** rollback: go back to "checkpoint" created before upgrade was started -
> hence newly created files are lost.
> Layout needs to be revisited if we want to support downgrade is some
> circumstances which we dont today. Here are use cases:
> * Some changes can support downgrade even though they was a change in layout
> since there is not real data loss but only loss of new functionality. E.g.
> when we added ACLs one could have downgraded - there is no data loss but you
> will lose the newly created ACLs. That is acceptable for a user since one
> does not expect to retain the newly added ACLs in an old version.
> * Some changes may lead to data-loss if the functionality was used. For
> example, the recent truncate will cause data loss if the functionality was
> actually used. Now one can tell admins NOT use such new such new features
> till the upgrade is finalized in which case one could potentially support
> downgrade.
> * A fairly fundamental change to layout where a downgrade is not possible but
> a rollback is. Say we change the layout completely from protobuf to something
> else. Another example is when HDFS moves to support partial namespace in
> memory - they is likely to be a fairly fundamental change in layout.
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