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From "Aaron T. Myers (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-3256) HDFS considers blocks under-replicated if topology script is configured with only 1 rack
Date Thu, 12 Apr 2012 02:22:17 GMT

     [ https://issues.apache.org/jira/browse/HDFS-3256?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Aaron T. Myers updated HDFS-3256:

    Attachment: HDFS-3256.patch

bq. Aaron, could you please describe the solution along with the patch.

Sure. In the original patch, the fix I had was just to check if only a single rack was configured
and not count a block as being on too-few racks if there in fact was only one rack.

However, here's an updated patch which should address the test failures.

There were two reasons for the test failures:
# There were 2 tests which asserted that blocks were under-replicated when only one rack was
configured, which is no longer the case.
# There was 1 test which had blocks for a single file spread across two racks, and asserted
that after all the nodes on one of the racks died, that blocks were then considered misreplicated.

Fixing #1 above just consisted of changing the tests to not assert that behavior.

Fixing #2 was a little more involved. I added a boolean to DatanodeManager to track whether
or not the cluster had _ever_ consisted of multiple racks. This is updated whenever a node
is added to the cluster. When a cluster goes from being sinlge-rack to multi-rack, we call
BlockManager#processMisReplicatedBlocks to make sure that we look for blocks which were not
considered under-replicated when there was only a single rack, that should now be on separate

Then, the check for whether or not a block is on enough racks becomes "is the desired replication
only 1, OR has there only ever been one rack configured in this cluster."
> HDFS considers blocks under-replicated if topology script is configured with only 1 rack
> ----------------------------------------------------------------------------------------
>                 Key: HDFS-3256
>                 URL: https://issues.apache.org/jira/browse/HDFS-3256
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 2.0.0
>            Reporter: Aaron T. Myers
>            Assignee: Aaron T. Myers
>         Attachments: HDFS-3256.patch, HDFS-3256.patch
> HDFS treats the mere presence of a topology script being configured as evidence that
there are multiple racks. If there is in fact only a single rack, the NN will try to place
the blocks on at least two racks, and thus blocks will be considered to be under-replicated.

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