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https://issues.apache.org/jira/browse/HDFS-5223?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14529313#comment-14529313
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Chris Nauroth commented on HDFS-5223:
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Hi [~atm].
bq. Seems like this approach would certainly help with the downgrade/rollback
issue, but wouldn't do much to make the upgrade itself easier.
That's correct. The rolling upgrade procedure still would be required. This
document/patch focuses on expanding the uses cases that can support downgrade.
bq. In general I think it'd be beneficial for HDFS to move toward a bit-set
denoting which features/op codes are enabled/disabled, much like Todd Lipcon
described earlier.
I share some of the concerns mentioned earlier about operational complexity.
https://issues.apache.org/jira/browse/HDFS-5223?focusedCommentId=13779177&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13779177
Complexity in HDFS often arises from combinations of its features rather than
individual features in isolation. If individual features can be toggled, then
no two HDFS instances running the same software version are really guaranteed
to be alike. This becomes another layer of troubleshooting required for a
technical support team. Testing the possible combinations of features on and
off becomes a combinatorial explosion that's difficult for a QA team to manage.
Aside from managing metadata upgrades, we've also found rolling upgrade to be
valuable because of the OOB ack propagated through write pipelines (HDFS-5583)
to tell clients to pause rather than aborting the connection. Even if it
wasn't required from a metadata standpoint, some users might continue to use
rolling upgrade to get this benefit, even within a minor release line where the
layout version hasn't changed. Considering that use case, I see value in
improving our ability to downgrade within the current rolling upgrade scheme.
If you prefer to keep the discussion here focused on building consensus around
feature flags, then I could potentially move this work to a separate jira where
it could move ahead independently. Let me know your thoughts. Thanks!
> 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-HDFS-Downgrade-Extended-Support.pdf,
> HDFS-5223.004.patch, HDFS-5223.005.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. (by
> sanjay on APril 7 2015)
> 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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