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From "Ming Ma (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-6178) Decommission on standby NN couldn't finish
Date Tue, 01 Apr 2014 03:29:14 GMT
Ming Ma created HDFS-6178:
-----------------------------

             Summary: Decommission on standby NN couldn't finish
                 Key: HDFS-6178
                 URL: https://issues.apache.org/jira/browse/HDFS-6178
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: namenode
            Reporter: Ming Ma


Currently decommissioning machines in HA-enabled cluster requires running refreshNodes in
both active and standby nodes. Sometimes decommissioning won't finish from standby NN's point
of view.  Here is the diagnosis of why it could happen.

Standby NN's blockManager manages blocks replication and block invalidation as if it is the
active NN; even though DNs will ignore block commands coming from standby NN. When standby
NN makes block operation decisions such as the target of block replication and the node to
remove excess blocks from, the decision is independent of active NN. So active NN and standby
NN could have different states. When we try to decommission nodes on standby nodes; such state
inconsistency might prevent standby NN from making progress. Here is an example.

Machine A
Machine B
Machine C
Machine D
Machine E
Machine F
Machine G
Machine H

1. For a given block, both active and standby have 5 replicas on machine A, B, C, D, E. So
both active and standby decide to pick excess nodes to invalidate.

Active picked D and E as excess DNs. After the next block reports from D and E, active NN
has 3 active replicas (A, B, C), 0 excess replica.

{noformat}
2014-03-27 01:50:14,410 INFO BlockStateChange: BLOCK* chooseExcessReplicates: (E:50010, blk_-5207804474559026159_121186764)
is added to invalidated blocks set
2014-03-27 01:50:15,539 INFO BlockStateChange: BLOCK* chooseExcessReplicates: (D:50010, blk_-5207804474559026159_121186764)
is added to invalidated blocks set
{noformat}

Standby pick C, E as excess DNs. Given DNs ignore commands from standby, After the next block
reports from C, D, E,  standby has 2 active replicas (A, B), 1 excess replica (C).

{noformat}
2014-03-27 01:51:49,543 INFO BlockStateChange: BLOCK* chooseExcessReplicates: (E:50010, blk_-5207804474559026159_121186764)
is added to invalidated blocks set
2014-03-27 01:51:49,894 INFO BlockStateChange: BLOCK* chooseExcessReplicates: (C:50010, blk_-5207804474559026159_121186764)
is added to invalidated blocks set
{noformat}


2. Machine A decomm request was sent to standby. Standby only had one live replica and picked
machine G, H as targets, but given standby commands was ignored by DNs, G, H remained in pending
replication queue until they are timed out. At this point, you have one decommissioning replica
(A), 1 active replica (B), one excess replica (C).
{noformat}
2014-03-27 04:42:52,258 INFO BlockStateChange: BLOCK* ask A:50010 to replicate blk_-5207804474559026159_121186764
to datanode(s) G:50010 H:50010
{noformat}

3. Machine A decomm request was sent to active NN. Active NN picked machine F as the target.
It finished properly. So active NN had 3 active replicas (B, C, F), one decommissioned replica
(A).

{noformat}
2014-03-27 04:44:15,239 INFO BlockStateChange: BLOCK* ask 10.42.246.110:50010 to replicate
blk_-5207804474559026159_121186764 to datanode(s) F:50010
2014-03-27 04:44:16,083 INFO BlockStateChange: BLOCK* addStoredBlock: blockMap updated: F:50010
is added to blk_-5207804474559026159_121186764 size 7100065
{noformat}

4. Standby NN picked up F as a new replica. Thus standby had one decommissioning replica (A),
2 active replicas (B, F), one excess replica (C). Standby NN kept trying to schedule replication
work, but DNs ignored the commands.

{noformat}
2014-03-27 04:44:16,084 INFO BlockStateChange: BLOCK* addStoredBlock: blockMap updated: F:50010
is added to blk_-5207804474559026159_121186764 size 7100065

2014-03-28 23:06:11,970 INFO org.apache.hadoop.hdfs.server.blockmanagement.BlockManager: Block:
blk_-5207804474559026159_121186764, Expected Replicas: 3, live replicas: 2, corrupt replicas:
0, decommissioned replicas: 1, excess replicas: 1, Is Open File: false, Datanodes having this
block: C:50010 B:50010 A:50010 F:50010 , Current Datanode: A:50010, Is current datanode decommissioning:
true
{noformat}



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