From 39c809b33fe27064277292b721639cbb144fac6a Mon Sep 17 00:00:00 2001 From: Zac Dover Date: Sat, 17 Feb 2024 15:28:24 +1000 Subject: [PATCH] doc/rados: add PG definition Add a definition of Placement Groups to doc/rados/operations/placement-groups.rst. Co-authored-by: Anthony D'Atri Signed-off-by: Zac Dover --- doc/rados/operations/placement-groups.rst | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/doc/rados/operations/placement-groups.rst b/doc/rados/operations/placement-groups.rst index d32feaf952357..f9b87528b5288 100644 --- 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 `_ -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: -- 2.39.5