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From "Steve Loughran (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-2486) Review issues with UnderReplicatedBlocks
Date Fri, 21 Oct 2011 15:12:33 GMT
Review issues with UnderReplicatedBlocks
----------------------------------------

                 Key: HDFS-2486
                 URL: https://issues.apache.org/jira/browse/HDFS-2486
             Project: Hadoop HDFS
          Issue Type: Task
          Components: name-node
    Affects Versions: 0.23.0
            Reporter: Steve Loughran
            Priority: Minor


Here are some things I've noted in the UnderReplicatedBlocks class that someone else should
review and consider if the code is correct. If not, they are easy to fix.

remove(Block block, int priLevel) is not synchronized, and as the inner classes are not, there
is a risk of race conditions there.

some of the code assumes that getPriority can return the value LEVEL, and if so does not attempt
to queue the blocks. As this return value is not currently possible, those checks can be removed.


The queue gives priority to blocks whose replication count is less than a third of its expected
count over those that are "normally under replicated". While this is good for ensuring that
files scheduled for large replication are replicated fast, it may not be the best strategy
for maintaining data integrity. For that it may be better to give whichever blocks have only
two replicas priority over blocks that may, for example, already have 3 out of 10 copies in
the filesystem.

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