HA: edit log should log to shared dirs before local dirs
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Key: HDFS-2874
URL: https://issues.apache.org/jira/browse/HDFS-2874
Project: Hadoop HDFS
Issue Type: Sub-task
Components: ha, name-node
Affects Versions: HA branch (HDFS-1623)
Reporter: Todd Lipcon
Priority: Critical
Currently, the NN logs its edits to each of its edits directories in sequence.
This can produce the following bad sequence:
- NN accumulates 100 edits (tx 1-100) in the buffer. Writes and syncs to local
drive, then crashes
- Failover occurs. SBN takes over at txid=1, since txid 1 never got writen.
- First NN restarts. It reads up to txid 100 from its local directories. It is
now "ahead" of the active NN with inconsistent state.
The solution is to write to the shared edits dir, and sync that, before writing
to any local drives.
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