[jira] [Created] (HDFS-4257) The ReplaceDatanodeOnFailure policies could have a forgiving option

["Harsh J (JIRA)" <ji...@apache.org>]

Harsh J created HDFS-4257:
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             Summary: The ReplaceDatanodeOnFailure policies could have a forgiving option
                 Key: HDFS-4257
                 URL: https://issues.apache.org/jira/browse/HDFS-4257
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: hdfs-client
    Affects Versions: 2.0.2-alpha
            Reporter: Harsh J
            Priority: Minor


Similar question has previously come over HDFS-3091 and friends, but the essential problem is: "Why can't I write to my cluster of 3 nodes, when I just have 1 node available at a point in time.".

The policies cover the 4 options, with {{Default}} being default:

{{Disable}} -> Disables the whole replacement concept by throwing out an error.
{{Never}} -> Never replaces a DN upon pipeline failures (not too desirable in many cases).
{{Default}} -> Replace based on a few conditions, but whose minimum never touches 1. We always fail if only one DN remains and none others can be added.
{{Always}} -> Replace no matter what. Fail if can't replace.

Would it not make sense to have an option similar to Always/Default, where despite _trying_, if it isn't possible to have > 1 DN in the pipeline, do not fail. I think that is what the former write behavior was, and what fit with the minimum replication factor allowed value.

Why is it grossly wrong to pass a write from a client for a block with just 1 remaining replica in the pipeline (the minimum of 1 grows with the replication factor demanded from the write), when replication is taken care of immediately afterwards? How often have we seen missing blocks arise out of allowing this + facing a big rack(s) failure or so?

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