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Date: Tue, 19 Aug 2014 05:56:21 +0000 (UTC)
From: "Tsz Wo Nicholas Sze (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Message-ID: <JIRA.12618462.1354397213394.30226.1408427781037@arcas>
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Subject: [jira] [Updated] (HDFS-4257) The ReplaceDatanodeOnFailure policies
 could have a forgiving option
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     [ https://issues.apache.org/jira/browse/HDFS-4257?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Tsz Wo Nicholas Sze updated HDFS-4257:
--------------------------------------

    Attachment: h4257_20140819.patch

h4257_20140819.patch: synced with trunk.

> 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
>         Attachments: h4257_20140325.patch, h4257_20140325b.patch, h4257_20140326.patch, h4257_20140819.patch
>
>
> 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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