Xiao Chen created HDFS-13788:
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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:
{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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