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From "Kihwal Lee (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8863) The remiaing space check in BlockPlacementPolicyDefault is flawed
Date Tue, 11 Aug 2015 13:58:46 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8863?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14681832#comment-14681832
] 

Kihwal Lee commented on HDFS-8863:
----------------------------------

bq.  it should just return current storage remaining space instead of get the maximum remaining
space of all storages
Datanodes only care about the storage type, so checking a particular storagewon't do any good.
It will just cause block placement to re-pick target more.

bq. Another issue, getBlocksScheduled is for storage type, not for per storage.
Tracking scheduled writes per storage is not going to solve the problem since datanodes are
free to choose any storage as long as the type matches. Trying to achieve precise accounting
will have diminishing return as there are uncertainties around actual storage being used,
blocks being abandoned, control loop delays (heartbeats), etc.

What if we let it check against storage type level sum and also make sure there is at least
one storage with enough space?  I actually had a version of patch that does just that.  I
will remove unused method and post the patch.

> The remiaing space check in BlockPlacementPolicyDefault is flawed
> -----------------------------------------------------------------
>
>                 Key: HDFS-8863
>                 URL: https://issues.apache.org/jira/browse/HDFS-8863
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 2.6.0
>            Reporter: Kihwal Lee
>            Assignee: Kihwal Lee
>            Priority: Critical
>              Labels: 2.6.1-candidate
>         Attachments: HDFS-8863.patch
>
>
> The block placement policy calls {{DatanodeDescriptor#getRemaining(StorageType}}}} to
check whether the block is going to fit. Since the method is adding up all remaining spaces,
namenode can allocate a new block on a full node. This causes pipeline construction failure
and {{abandonBlock}}. If the cluster is nearly full, the client might hit this multiple times
and the write can fail permanently.



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