doc: add "to Hammer" upgrade section

Fixes: http://tracker.ceph.com/issues/17534
Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/doc/install/upgrading-ceph.rst b/doc/install/upgrading-ceph.rst
index b97a6d30bcb1310b1aa5099d096bb2c7126fc9cf..d0ff8661158cd49a487607fa60998db2bc3bdb27 100644 (file)
--- a/doc/install/upgrading-ceph.rst
+++ b/doc/install/upgrading-ceph.rst
@@ -473,6 +473,20 @@ details. To upgrade from a post-Emperor point release, see the `Firefly release
 notes`_ for details.
 
 
+Upgrading to Hammer
+===================
+
+When upgrading from v0.94.6 (and lower) to Hammer (v0.94.7 and higher), you need
+to upgrade the OSD **before** the monitors.
+
+Due to changes in encoding of osdmap in v0.94.7, upgrading monitors before OSDs
+can lead to serious performance issues on large clusters that contain hundreds of
+OSDs. So, we should upgrade OSDs first to work around this issue. It is fixed
+in Jewel and higher versions of Ceph. So you can follow the general steps when
+upgrading from Hammer to Jewel, or from Jewel to Kraken.
+
+
+
 Ceph Config File Changes
 ------------------------
 
@@ -748,4 +762,4 @@ Under those directories, the keyring should be in a file named ``keyring``.
 .. _Operating a Cluster: ../../rados/operations/operating
 .. _Monitoring a Cluster: ../../rados/operations/monitoring
 .. _Firefly release notes: ../../release-notes/#v0-80-firefly
-.. _release notes: ../../release-notes
\ No newline at end of file
+.. _release notes: ../../release-notes