rolling upgrade without downtime

[Dmitry Sivachenko <tr...@gmail.com>]

Hello!

I am trying to perform a rolling upgrade of a cluster running hadoop-2.4.1 without downtime following procedure described at
http://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-hdfs/HdfsRollingUpgrade.html

The first command to execute is:

% hdfs dfsadmin -rollingUpgrade prepare
PREPARE rolling upgrade ...
rollingUpgrade: Safe mode should be turned ON in order to create namespace image.

So according to this message I must switch HDFS to safe mode first.

1) The documentation should be adjusted to mention that fact;
2) switching to safe mode will imply some restrictions to using the cluster, so it is not *without downtime*

Or may be I am missing something?

Thanks in advance!