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From "Andrew Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8776) Decom manager should not be active on standby
Date Thu, 16 Jul 2015 19:02:10 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8776?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14630185#comment-14630185

Andrew Wang commented on HDFS-8776:

Related question to Ming's, this means node state will could flip back to decom in progress
from decommissioned on failover right? I guess this can happen anyway, but is less likely
in the current world.

I'm also curious if this is ameliorated by the decom manager rewrite in HDFS-7411, since in-progress
scans are incremental. We could also invest some more effort making it fully incremental if
that would help.

> Decom manager should not be active on standby
> ---------------------------------------------
>                 Key: HDFS-8776
>                 URL: https://issues.apache.org/jira/browse/HDFS-8776
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: namenode
>    Affects Versions: 2.6.0
>            Reporter: Daryn Sharp
>            Assignee: Daryn Sharp
> The decommission manager should not be actively processing on the standby.
> The decomm manager goes through the costly computation for determining every block on
the node requires replication yet doesn't queue them for replication - because it's in standby.
The decomm manager is holding the namesystem write lock, causing DNs to timeout on heartbeats
or IBRs, NN purges the call queue of timed out clients, NN processes some heartbeats/IBRs
before the decomm manager locks up the namesystem again. Nodes attempting to register will
be sending full BRs which are more costly to send and discard than a heartbeat.
> If a failover is required, the standby will likely have to struggle very hard to not
GC while "catching up" on its queued IBRs while DNs continue to fill the call queue and time

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