nkeywal created HDFS-3705:
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Summary: Add the possibility to mark a node as 'low priority' for
read in the DFSClient
Key: HDFS-3705
URL: https://issues.apache.org/jira/browse/HDFS-3705
Project: Hadoop HDFS
Issue Type: Improvement
Components: hdfs client
Affects Versions: 2.0.0-alpha, 1.0.3
Reporter: nkeywal
This has been partly discussed in HBASE-6435.
The DFSClient includes a 'bad nodes' management for reads and writes.
Sometimes, the client application already know that some deads are dead or
likely to be dead.
An example is the 'HBase Write-Ahead-Log': when HBase reads this file, it knows
that the HBase regionserver died, and it's very likely that the box died so the
datanode on the same box is dead as well. This is actually critical, because:
- it's the hbase recovery that reads these log files
- if we read them it means that we lost a box, so we have 1 dead replica out
the the 3.
- for all files read, we have 33% of chance to go to the dead datanode
- as the box just died, we're very likely to get a timeout exception so we're
delaying the hbase recovery by 1 minute. For HBase, it means that the data is
not available during this minute.
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