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From "Xiao Chen (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-13788) Update EC documentation about rack fault tolerance
Date Thu, 02 Aug 2018 16:46:00 GMT
Xiao Chen created HDFS-13788:

             Summary: Update EC documentation about rack fault tolerance
                 Key: HDFS-13788
                 URL: https://issues.apache.org/jira/browse/HDFS-13788
             Project: Hadoop HDFS
          Issue Type: Task
          Components: documentation, erasure-coding
    Affects Versions: 3.0.0
            Reporter: Xiao Chen
            Assignee: Kitti Nanasi

>From http://hadoop.apache.org/docs/r3.0.0/hadoop-project-dist/hadoop-hdfs/HDFSErasureCoding.html:
For rack fault-tolerance, it is also important to have at least as many racks as the configured
EC stripe width. For EC policy RS (6,3), this means minimally 9 racks, and ideally 10 or 11
to handle planned and unplanned outages. For clusters with fewer racks than the stripe width,
HDFS cannot maintain rack fault-tolerance, but will still attempt to spread a striped file
across multiple nodes to preserve node-level fault-tolerance.
Theoretical minimum is 3 racks, and ideally 9 or more, so the document should be updated.

(I didn't check timestamps, but this is probably due to {{BlockPlacementPolicyRackFaultTolerant}}
isn't completely done when HDFS-9088 introduced this doc. Later there's also examples in {{TestErasureCodingMultipleRacks}}
to test this explicitly.)

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