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From "Harsh J (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4257) The ReplaceDatanodeOnFailure policies could have a forgiving option
Date Sun, 02 Dec 2012 04:31:59 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13508143#comment-13508143
] 

Harsh J commented on HDFS-4257:
-------------------------------

Thanks for the comment Nicholas!

I feel having an policy-option 'TOLERATE' (or similar) would be much cleaner than a direct
string toggle, if it is possible to implement it this way, thoughts?

Would we also be making it default (to go back to older behavior), or be continuing with 'DEFAULT'?
                
> 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: New Feature
>          Components: hdfs-client
>    Affects Versions: 2.0.2-alpha
>            Reporter: Harsh J
>            Assignee: Tsz Wo (Nicholas), SZE
>            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 (at
the server) or acts as {{Never}} at the client.
> {{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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