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From "Andy Isaacson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3931) TestDatanodeBlockScanner#testBlockCorruptionPolicy2 is broken
Date Thu, 20 Sep 2012 23:11:07 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3931?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13460038#comment-13460038
] 

Andy Isaacson commented on HDFS-3931:
-------------------------------------

bq. I'm surprised that there's a scenario where 20 is insufficient but it still eventually
does get replicated correctly.
The scenario is
- NN requests replication, puts block on pending replication queue.
- requested replication source is corrupt, replication fails.
- eventually NN times out pending replication queue and tries again. Block scanner has run
and marked replica as corrupt so eventually we are guaranteed the replication source is not
corrupt.

Without cranking DFS_NAMENODE_REPLICATION_PENDING_TIMEOUT_SEC_KEY down, we fail 1/2 tries.
After speeding up pending replication timeouts, we fail 1/2*1/2*1/2... until the waitReplication
loop bails. If the blockScanner has a chance to run and mark replicas corrupt, and after blockScanner
finishes the replication queue has time to run, then we succeed reliably.

Without increasing the timeout from 20 seconds to 40 seconds, the test fails 1/3 times.

bq. Why is restarting the datanodes ITERATIONS times necessary?

As the test is currently structured, we're polling to notice that the corrupt replica count
has increased. With the fixed blockscanner from HDFS-3828, the test fails to hit the timing
window to notice corrupt replicas because the blockscanner sometimes (1/20 in my tests IIRC)
wins a race and the NN deletes the corrupt replica before the test can poll it.  If this happens,
we have to go back and re-try the whole "corrupt two replicas, restart their DNs, poll" loop.

So it's not "restarting the datanodes" that we are retrying here.  The thing we're retrying
is "corrupt the replicas, restart the DNs, watch the corrupt count to verify it changed".

                
> TestDatanodeBlockScanner#testBlockCorruptionPolicy2 is broken
> -------------------------------------------------------------
>
>                 Key: HDFS-3931
>                 URL: https://issues.apache.org/jira/browse/HDFS-3931
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: test
>    Affects Versions: 2.0.0-alpha
>            Reporter: Eli Collins
>            Assignee: Andy Isaacson
>         Attachments: hdfs3931-1.txt, hdfs3931-2.txt, hdfs3931-3.txt, hdfs3931.txt
>
>
> Per Andy's comment on HDFS-3902:
> TestDatanodeBlockScanner still fails about 1/5 runs in testBlockCorruptionRecoveryPolicy2.
That's due to a separate test issue also uncovered by HDFS-3828.
> The failure scenario for this one is a bit more tricky. I think I've captured the scenario
below:
> - The test corrupts 2/3 replicas.
> - client reports a bad block.
> - NN asks a DN to re-replicate, and randomly picks the other corrupt replica.
> - DN notices the incoming replica is corrupt and reports it as a bad block, but does
not inform the NN that re-replication failed.
> - NN keeps the block on pendingReplications.
> - BP scanner wakes up on both DNs with corrupt blocks, both report corruption. NN reports
both as duplicates, one from the client and one from the DN report above.
> since block is on pendingReplications, NN does not schedule another replication.

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