doc/rados: add PG definition

Add a definition of Placement Groups to
doc/rados/operations/placement-groups.rst.

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>

diff --git a/doc/rados/operations/placement-groups.rst b/doc/rados/operations/placement-groups.rst
index d32feaf9523573d5110be66e639d59756c27e3bf..f9b87528b52884e081e337601def9faef9fbeb58 100644 (file)
--- a/doc/rados/operations/placement-groups.rst
+++ b/doc/rados/operations/placement-groups.rst
@@ -4,9 +4,19 @@
  Placement Groups
 ==================
 
+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 RADOS objects. A cluster that has a larger number of
+placement groups (for example, 150 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.
+
 See Sage Weil's blog post `New in Nautilus: PG merging and autotuning
 <https://ceph.io/en/news/blog/2019/new-in-nautilus-pg-merging-and-autotuning/>`_
-for information about the relationship of placement groups to pools and to
+for more information about the relationship of placement groups to pools and to
 objects.
 
 .. _pg-autoscaler: