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https://issues.apache.org/jira/browse/HDFS-6800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14116257#comment-14116257
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James Thomas commented on HDFS-6800:
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I tested that rolling upgrade to and rolling upgrade rollback from DN layout
version -56 (HDFS-6482) works without data loss. I also validated the first,
third, and fourth bullets above (the fourth by doing a rolling upgrade to the
original software version). These tests were run on a single node with a single
NN (no HA) and a single DN, and one or two files were added to the DFS in each
case.
The second bullet unfortunately raises an issue with this patch -- we can
encounter the following scenario:
(1) We start with DN software version -55 and initiate a rolling upgrade to
version -56
(2) We delete some blocks, and they are moved to trash
(3) We roll back to DN software version -55 using the {{-rollback}} flag --
since we are running the old code (prior to this patch), we will restore the
{{previous}} directory but will not delete the trash
(4) We append to some of the blocks that were deleted in step 2
(5) We then restart a DN that contains blocks that were appended to -- since
the trash still exists, it will be restored at this point, the appended-to
blocks will be overwritten, and we will lose the appended data
So I think we need to avoid writing anything to the trash directory if we have
a {{previous}} directory. I have not thought through this completely.
[~arpitagarwal], any thoughts?
> Support Datanode layout changes with rolling upgrade
> ----------------------------------------------------
>
> Key: HDFS-6800
> URL: https://issues.apache.org/jira/browse/HDFS-6800
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 2.6.0
> Reporter: Colin Patrick McCabe
> Assignee: James Thomas
> Fix For: 3.0.0, 2.6.0
>
> Attachments: HDFS-6800.2.patch, HDFS-6800.3.patch, HDFS-6800.4.patch,
> HDFS-6800.5.patch, HDFS-6800.6.patch, HDFS-6800.7.patch, HDFS-6800.8.patch,
> HDFS-6800.patch
>
>
> We need to handle attempts to rolling-upgrade the DataNode to a new storage
> directory layout.
> One approach is to disallow such upgrades. If we choose this approach, we
> should make sure that the system administrator gets a helpful error message
> and a clean failure when trying to use rolling upgrade to a version that
> doesn't support it. Based on the compatibility guarantees described in
> HDFS-5535, this would mean that *any* future DataNode layout changes would
> require a major version upgrade.
> Another approach would be to support rolling upgrade from an old DN storage
> layout to a new layout. This approach requires us to change our
> documentation to explain to users that they should supply the {{\-rollback}}
> command on the command-line when re-starting the DataNodes during rolling
> rollback. Currently the documentation just says to restart the DataNode
> normally.
> Another issue here is that the DataNode's usage message describes rollback
> options that no longer exist. The help text says that the DN supports
> {{\-rollingupgrade rollback}}, but this option was removed by HDFS-6005.
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