doc/glossary: add "Placement Groups" definition

Add a definition of "Placement Groups" to the Glossary.

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 9f37ea651f9ee2c51e0705b9b58ed356f1bc56e6)

diff --git a/doc/glossary.rst b/doc/glossary.rst
index ea72ae8c268d611e98ba3fabd21a52039d65cd14..9f685e42c7fb6495f865937c02283f84ef7aa7db 100644 (file)
--- a/doc/glossary.rst
+++ b/doc/glossary.rst
@@ -311,6 +311,20 @@
                 state of a multi-site configuration. When the period is updated,
                 the "epoch" is said thereby to have been changed.
 
+        Placement Groups (PGs)
+                Placement groups (PGs) are subsets of each logical Ceph pool.
+                Placement groups perform the function of placing objects (as a
+                group) into OSDs. Ceph manages data internally at
+                placement-group granularity: this scales better than would
+                managing individual (and therefore more numerous) RADOS
+                objects. A cluster that has a larger number of placement groups
+                (for example, 100 per OSD) is better balanced than an otherwise
+                identical cluster with a smaller number of placement groups. 
+                
+                Ceph's internal RADOS objects are each mapped to a specific
+                placement group, and each placement group belongs to exactly
+                one Ceph pool. 
+
        :ref:`Pool<rados_pools>`
                A pool is a logical partition used to store objects.