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From "Derek Dagit (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-4366) Block Replication Policy Implementation May Skip Higher-Priority Blocks for Lower-Priority Blocks
Date Tue, 08 Jan 2013 23:10:13 GMT
Derek Dagit created HDFS-4366:

             Summary: Block Replication Policy Implementation May Skip Higher-Priority Blocks
for Lower-Priority Blocks
                 Key: HDFS-4366
                 URL: https://issues.apache.org/jira/browse/HDFS-4366
             Project: Hadoop HDFS
          Issue Type: Bug
    Affects Versions: 0.23.5, 1.1.1, 3.0.0
            Reporter: Derek Dagit
            Assignee: Derek Dagit

In certain cases, higher-priority under-replicated blocks can be skipped by the replication
policy implementation.  The current implementation maintains, for each priority level, an
index into a list of blocks that are under-replicated.  Together, the lists compose a priority
queue (see note later about branch-0.23).  In some cases when blocks are removed from a list,
the caller (BlockManager) properly handles the index into the list from which it removed a
block.  In some other cases, the index remains stationary while the list changes.  Whenever
this happens, and the removed block happened to be at or before the index, the implementation
will skip over a block when selecting blocks for replication work.

In situations when entire racks are decommissioned, leading to many under-replicated blocks,
loss of blocks can occur.

Background: HDFS-1765

This patch to trunk greatly improved the state of the replication policy implementation. 
Prior to the patch, the following details were true:
	* The block "priority queue" was no such thing: It was really set of trees that held blocks
in natural ordering, that being by the blocks ID, which resulted in iterator walks over the
blocks in pseudo-random order.
	* There was only a single index into an iteration over all of the blocks...
	* ... meaning the implementation was only successful in respecting priority levels on the
first pass.  Overall, the behavior was a round-robin-type scheduling of blocks.

After the patch
	* A proper priority queue is implemented, preserving log(n) operations while iterating over
blocks in the order added.
	* A separate index for each priority is key is kept...
	* ... allowing for processing of the highest priority blocks first regardless of which priority
had last been processed.

The change was suggested for branch-0.23 as well as trunk, but it does not appear to have
been pulled in.

The problem:

Although the indices are now tracked in a better way, there is a synchronization issue since
the indices are managed outside of methods to modify the contents of the queue.

Removal of a block from a priority level without adjusting the index can mean that the index
then points to the block after the block it originally pointed to.  In the next round of scheduling
for that priority level, the block originally pointed to by the index is skipped.

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