Github user steveloughran commented on the issue:
https://github.com/apache/spark/pull/15648
LGTM, as the javadocs say *If true check only for Active NNs status, else
check first NN's status*. But I don't know enough about HDFS HA to be
It'll check the first NN, if that is on standby *and stale reads are not
allowed( it'll log at error (HDFS-3477 proposes downgrading that), and throw an
exception the url
[https://s.apache.org/sbnn-error](https://s.apache.org/sbnn-error). If someone
sets `dfs.ha.allow.stale.reads"` then they get the old safe mode state; there's
nothing that can be done there.
Where my knowledge of HDFS-HA fails is what happens then; Does the RPC
client try another NN? Or just it just fail? Maybe @liuml07 could assist there.
The method went in with Hadoop 2.0.3 alpha in
[HDFS-3507](https://issues.apache.org/jira/browse/HDFS-3507), so will be across
the whole of the Hadoop 2.x line. The enum used did change in 2015, with
HDFS-4015; adding `SAFEMODE_FORCE_EXIT`; that action must be avoided. Luckily,
the history server isn't trying to exit safemode.
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