Stephen O'Donnell created HDFS-14861:
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Summary: Reset LowRedundancyBlocks Iterator periodically
Key: HDFS-14861
URL: https://issues.apache.org/jira/browse/HDFS-14861
Project: Hadoop HDFS
Issue Type: Improvement
Components: namenode
Affects Versions: 3.3.0
Reporter: Stephen O'Donnell
Assignee: Stephen O'Donnell
When the namenode needs to schedule blocks for reconstruction, the blocks are
placed into the neededReconstruction object in the BlockManager. This is an
instance of LowRedundancyBlocks, which maintains a list of priority queues
where the blocks are held until they are scheduled for reconstruction /
replication.
Every 3 seconds, by default, a number of blocks are retrieved from
LowRedundancyBlocks. The method LowRedundancyBlocks.chooseLowRedundancyBlocks()
is used to retrieve the next set of blocks using a bookmarked iterator. Each
call to this method moves the iterator forward. The number of blocks retrieved
is governed by the formula:
number_of_live_nodes * dfs.namenode.replication.work.multiplier.per.iteration
(default 2)
Then the namenode attempts to schedule those blocks on datanodes, but each
datanode has a limit of how many blocks can be queued against it (controlled by
dfs.namenode.replication.max-streams) so all of the retrieved blocks may not be
scheduled. There may be other block availability reasons the blocks are not
scheduled too.
As the iterator in chooseLowRedundancyBlocks() always moves forward, the blocks
which were not scheduled are not retried until the end of the queue is reached
and the iterator is reset.
If the replication queue is very large (eg several nodes are being
decommissioned) or if blocks are being continuously added to the replication
queue (eg nodes decommission using the proposal in HDFS-14854) it may take a
very long time for the iterator to be reset to the start.
The result of this, could be a few blocks for a decommissioning or entering
maintenance mode node getting left behind and it taking many hours or even days
for them to be retried, and this could stop decommission completing.
With this Jira, I would like to suggest we reset the iterator after a
configurable number of calls to chooseLowRedundancyBlocks() so any left behind
blocks are retried.
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