CDH downgrade from 5.5 to 5.3

[#1]

Hi,

Does somebody happen to know whether CDH can be downgraded from 5.5.x to 5.3/5.4?

I tried to use old parcel but then HDFS namenodes fails to start due to "Unexpected version of storage directory .../dfs/nn".

Also I found the post https://community.cloudera.com/t5/Cloudera-Manager-Installation/Downgrade-Procedure/td-p/25430 where it was said that Cloudera didn't support downgrade operations. But maybe somebody met (un)official instructions how to do it manually?

Thank you in advance!

[#2]

Out of curiosity, why do you want to downgrade? Perhaps there is another way to reach your overall goal.

[#3]

I'm going to upgrade production cluster so would like to have a possibility to restore old CDH in case of some critical issues. Cluster is located on hardware servers so I cannot use snapshots.

[#4 · Accepted solution]

@mstepanov here's the background, if I remember correctly:

1) the reason rollbacks can be impossible in certain circumstances in HDFS is because sometimes the metadata is changed between versions in non-backward-compatible ways.  Basically the namenode's fsimage and each HDFS block in the cluster get re-encoded with new bits of metadata that weren't available and therefore have no meaning in the older CDH version.

2) we don't always update the HDFS metadata in non-backward compatible ways between releases, but when we do it's usually major releases (eg. CDH4 to CDH5).  

3) If no HDFS metadata changes were made between your original and upgraded version, rollback is possible in limited fashion.

4) Even if metadata changes were made, you can still rollback IF you haven't finalized the upgrade yet.  See this doc for rollback procedures between CDH5 and CDH4.

Your best bet is to have a solid upgrade plan and test it thoroughly (UAT, QA, and regression) in a similarly configured staging environment prior to upgrade.

[#5]

Clint, thank you for this information!