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From "Kitti Nanasi (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-13788) Update EC documentation about rack fault tolerance
Date Mon, 13 Aug 2018 09:35:00 GMT

     [ https://issues.apache.org/jira/browse/HDFS-13788?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Kitti Nanasi updated HDFS-13788:
    Status: Patch Available  (was: Open)

> 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
>            Priority: Major
>         Attachments: HDFS-13788.001.patch
> From http://hadoop.apache.org/docs/r3.0.0/hadoop-project-dist/hadoop-hdfs/HDFSErasureCoding.html:
> {quote}
> 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.
> {quote}
> 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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