Allen Wittenauer commented on HDFS-10999:

Given that the fsck output is the *only* way to get some pieces of information, 
changing fsck is almost always a major, ops breaking event.  Coupling that with 
pretty much breaking metrics collection... There is no "supposed" here: this is 
very very incompatible and will cause admins to burn Apache Hadoop conferences 
to the ground in their anger if we aren't careful.

That said, I can empathize with the EC folks.  'under replicated' doesn't 
really cover the state of a block with a missing reconstructable chunk.  But 
I'm not sure that 'low redundancy' necessarily conveys the state of a non-EC 
block either. If I'm not running EC at all, it comes across as a gratuitous 
change.  I need to think more about this, to be honest.

But some questions first:

I'm trying to remember, is it possible to have EC and non-EC blocks in a file 
system?  If not, what about in the future? Are we actually trying to shoe horn 
two separate measurements into the same metric here?  Is there a situation 
where having both under replicated and low redundancy blocks makes sense?  How 
does the storage policy interact with a change like this?

> Use more generic "low redundancy" blocks instead of "under replicated" blocks
> -----------------------------------------------------------------------------
>                 Key: HDFS-10999
>                 URL: https://issues.apache.org/jira/browse/HDFS-10999
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: erasure-coding
>    Affects Versions: 3.0.0-alpha1
>            Reporter: Wei-Chiu Chuang
>            Assignee: Yuanbo Liu
>              Labels: supportability
> Per HDFS-9857, it seems in the Hadoop 3 world, people prefer the more generic 
> term "low redundancy" to the old-fashioned "under replicated". But this term 
> is still being used in messages in several places, such as web ui, dfsadmin 
> and fsck. We should probably change them to avoid confusion.
> File this jira to discuss it.

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