doc: explain about pgp num

Explain the significance of pgp num & how it differs from pg num.

Fixes: #10035
Reported-by: Loic Dachary <loic@dachary.org>
Signed-off-by: Abhishek Lekshmanan <abhishek.lekshmanan@ril.com>

diff --git a/doc/rados/operations/placement-groups.rst b/doc/rados/operations/placement-groups.rst
index fe99269e6e45506d5d8a4185715bf82d5387f97c..2d77df6b97bf26bbc95113110a0f1f001264cc67 100644 (file)
--- a/doc/rados/operations/placement-groups.rst
+++ b/doc/rados/operations/placement-groups.rst
@@ -310,10 +310,14 @@ placement groups, execute the following::
         ceph osd pool set {pool-name} pg_num {pg_num}
 
 Once you increase the number of placement groups, you must also
-increase the number of placement groups for placement (``pgp_num``) before your
-cluster will rebalance. The ``pgp_num`` should be equal to the ``pg_num``.
-To increase the number of placement groups for placement, execute the
-following::
+increase the number of placement groups for placement (``pgp_num``)
+before your cluster will rebalance. The ``pgp_num`` will be the number of
+placement groups that will be considered for placement by the CRUSH
+algorithm. Increasing ``pg_num`` splits the placement groups but data
+will not be migrated to the newer placement groups until placement
+groups for placement, ie. ``pgp_num`` is increased. The ``pgp_num``
+should be equal to the ``pg_num``.  To increase the number of
+placement groups for placement, execute the following::
 
         ceph osd pool set {pool-name} pgp_num {pgp_num}
 

diff --git a/doc/rados/operations/pools.rst b/doc/rados/operations/pools.rst
index 95ed7a6742aee9d3df5bb3239ce1790f78c43281..46793cff5989ccf2ec3037cbf08469c82cfd6ddf 100644 (file)
--- a/doc/rados/operations/pools.rst
+++ b/doc/rados/operations/pools.rst
@@ -53,6 +53,8 @@ Create a Pool
 Before creating pools, refer to the `Pool, PG and CRUSH Config Reference`_.
 Ideally, you should override the default value for the number of placement
 groups in your Ceph configuration file, as the default is NOT ideal.
+For details on placement group numbers refer to `setting the number of placement groups`_
+
 For example:: 
 
        osd pool default pg num = 100
@@ -568,3 +570,4 @@ a size of 3).
 
 .. _Pool, PG and CRUSH Config Reference: ../../configuration/pool-pg-config-ref
 .. _Bloom Filter: http://en.wikipedia.org/wiki/Bloom_filter
+.. _setting the number of placement groups: ../placement-groups#set-the-number-of-placement-groups