Overview

PG = “placement group”. When placing data in the cluster, objects aremapped into PGs, and those PGs are mapped onto OSDs. We use theindirection so that we can group objects, which reduces the amount ofper-object metadata we need to keep track of and processes we need torun (it would be prohibitively expensive to track eg the placementhistory on a per-object basis). Increasing the number of PGs canreduce the variance in per-OSD load across your cluster, but each PGrequires a bit more CPU and memory on the OSDs that are storing it. Wetry and ballpark it at 100 PGs/OSD, although it can vary widelywithout ill effects depending on your cluster. You hit a bug in how wecalculate the initial PG number from a cluster description.

There are a couple of different categories of PGs; the 6 that exist(in the original emailer’sceph-s output) are “local” PGs whichare tied to a specific OSD. However, those aren’t actually used in astandard Ceph configuration.