Mike Percy created HDFS-5131:
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Summary: Need a DEFAULT-like pipeline recovery policy that works
for writers that flush
Key: HDFS-5131
URL: https://issues.apache.org/jira/browse/HDFS-5131
Project: Hadoop HDFS
Issue Type: Improvement
Affects Versions: 2.0.6-alpha
Reporter: Mike Percy
The Hadoop 2 pipeline-recovery mechanism currently has four policies: DISABLE
(never do recovery), NEVER (never do recovery unless client asks for it),
ALWAYS (block until we have recovered the write pipeline to minimum replication
levels), and DEFAULT (try to do ALWAYS, but use a heuristic to "give up" and
allow writers to continue if not enough datanodes are available to recover the
pipeline).
The big problem with default is that it specifically falls back to ALWAYS
behavior if a client calls hflush(). On its face, it seems like a reasonable
thing to do, but in practice this means that clients like Flume (as well as, I
assume, HBase) just block when the cluster is low on datanodes.
In order to work around this issue, the easiest thing to do today is set the
policy to NEVER when using Flume to write to the cluster. But obviously that's
not ideal.
I believe what clients like Flume need is an additional policy which
essentially uses the heuristic logic used by DEFAULT even in cases where
long-lived writers call hflush().
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