Ming Ma created HDFS-6178:
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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.
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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
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