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From "Vinod Kumar Vavilapalli (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-7742) favoring decommissioning node for replication can cause a block to stay underreplicated for long periods
Date Wed, 02 Sep 2015 02:08:47 GMT

     [ https://issues.apache.org/jira/browse/HDFS-7742?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Vinod Kumar Vavilapalli updated HDFS-7742:
    Target Version/s: 2.7.0, 3.0.0  (was: 3.0.0, 2.7.0)
       Fix Version/s: 2.6.1

[~sjlee0] backported this to 2.6.1, the patch applies cleanly.

I just pushed the commit to 2.6.1 after running compilation and TestBlockManager which changed
in the patch.

> favoring decommissioning node for replication can cause a block to stay underreplicated
for long periods
> --------------------------------------------------------------------------------------------------------
>                 Key: HDFS-7742
>                 URL: https://issues.apache.org/jira/browse/HDFS-7742
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: namenode
>    Affects Versions: 2.6.0
>            Reporter: Nathan Roberts
>            Assignee: Nathan Roberts
>              Labels: 2.6.1-candidate
>             Fix For: 2.7.0, 2.6.1
>         Attachments: HDFS-7742-v0.patch
> When choosing a source node to replicate a block from, a decommissioning node is favored.
The reason for the favoritism is that decommissioning nodes aren't servicing any writes so
in-theory they are less loaded.
> However, the same selection algorithm also tries to make sure it doesn't get "stuck"
on any particular node:
> {noformat}
>       // switch to a different node randomly
>       // this to prevent from deterministically selecting the same node even
>       // if the node failed to replicate the block on previous iterations
> {noformat}
> Unfortunately, the decommissioning check is prior to this randomness so the algorithm
can get stuck trying to replicate from a decommissioning node. We've seen this in practice
where a decommissioning datanode was failing to replicate a block for many days, when other
viable replicas of the block were available.
> Given that we limit the number of streams we'll assign to a given node (default soft
limit of 2, hard limit of 4), It doesn't seem like favoring a decommissioning node has significant
benefit. i.e. when there is significant replication work to do, we'll quickly hit the stream
limit of the decommissioning nodes and use other nodes in the cluster anyway; when there isn't
significant replication work then in theory we've got plenty of replication bandwidth available
so choosing a decommissioning node isn't much of a win.
> I see two choices:
> 1) Change the algorithm to still favor decommissioning nodes but with some level of randomness
that will avoid always selecting the decommissioning node
> 2) Remove the favoritism for decommissioning nodes
> I prefer #2. It simplifies the algorithm, and given the other throttles we have in place,
I'm not sure there is a significant benefit to selecting decommissioning nodes. 

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