Andy Isaacson created HDFS-4253:
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Summary: block replica reads get hot-spots due to
NetworkTopology#pseudoSortByDistance
Key: HDFS-4253
URL: https://issues.apache.org/jira/browse/HDFS-4253
Project: Hadoop HDFS
Issue Type: Bug
Affects Versions: 2.0.2-alpha, 3.0.0
Reporter: Andy Isaacson
Assignee: Andy Isaacson
When many nodes (10) read from the same block simultaneously, we get asymmetric
distribution of read load. This can result in slow block reads when one
replica is serving most of the readers and the other replicas are idle. The
busy DN bottlenecks on its network link.
This is especially visible with large block sizes and high replica counts (I
reproduced the problem with {{-Ddfs.block.size=4294967296}} and replication 5),
but the same behavior happens on a small scale with normal-sized blocks and
replication=3.
The root of the problem is in {{NetworkTopology#pseudoSortByDistance}} which
explicitly does not try to spread traffic among replicas in a given rack -- it
only randomizes usage for off-rack replicas.
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