FAQ
Corrupt replicas are not tracked correctly through block report from DN
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Key: HDFS-900
URL: https://issues.apache.org/jira/browse/HDFS-900
Project: Hadoop HDFS
Issue Type: Bug
Affects Versions: 0.22.0
Reporter: Todd Lipcon
Priority: Critical
Attachments: log-commented, to-reproduce.patch

This one is tough to describe, but essentially the following order of events is seen to occur:

# A client marks one replica of a block to be corrupt by telling the NN about it
# Replication is then scheduled to make a new replica of this node
# The replication completes, such that there are now 3 good replicas and 1 corrupt replica
# The DN holding the corrupt replica sends a block report. Rather than telling this DN to delete the node, the NN instead marks this as a new *good* replica of the block, and schedules deletion on one of the good replicas.

I don't know if this is a dataloss bug in the case of 1 corrupt replica with dfs.replication=2, but it seems feasible. I will attach a debug log with some commentary marked by '============>', plus a unit test patch which I can get to reproduce this behavior reliably. (it's not a proper unit test, just some edits to an existing one to show it)

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postedJan 14, '10 at 12:43a
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