From: Zac Dover Date: Sat, 17 Feb 2024 05:28:24 +0000 (+1000) Subject: doc/rados: add PG definition X-Git-Tag: testing/wip-pdonnell-testing-20240430.123648-reef-debug~242^2 X-Git-Url: http://git.apps.os.sepia.ceph.com/?a=commitdiff_plain;h=2406b501ad3d98dcddecdfe00af1d46661b5c94c;p=ceph-ci.git 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 (cherry picked from commit 39c809b33fe27064277292b721639cbb144fac6a) --- diff --git a/doc/rados/operations/placement-groups.rst b/doc/rados/operations/placement-groups.rst index 2e2e385a516..ab1fefe83cd 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: