[
https://issues.apache.org/jira/browse/HDFS-8075?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14483602#comment-14483602
]
Sanjay Radia commented on HDFS-8075:
------------------------------------
Here is proposal:
* Add a *second* layout version field (call it "compatible-layout-version")
that states which version can safely read the image without data-loss.
** The ACL example will state that the previous version can safely read the
image without data loss.
** Truncate example is tricky: one can safely downgrade if the truncate
operation was not used. We could add code to not allow such new features till
finalize is done. Or we could say don't support downgrade for such a feature
and simply mark the compatible-layout-version accordingly.
> Revist layout version
> ---------------------
>
> Key: HDFS-8075
> URL: https://issues.apache.org/jira/browse/HDFS-8075
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: HDFS
> Affects Versions: 2.6.0
> Reporter: Sanjay Radia
>
> 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.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
