Chapter 5. Pools, placement groups, and CRUSH configuration

As a storage administrator, you can choose to use the Red Hat Ceph Storage default options for pools, placement groups, and the CRUSH algorithm or customize them for the intended workload.

5.1. Prerequisites

  • Installation of the Red Hat Ceph Storage software.

5.2. Pools placement groups and CRUSH

When you create pools and set the number of placement groups for the pool, Ceph uses default values when you do not specifically override the defaults.


Red Hat recommends overriding some of the defaults. Specifically, set a pool’s replica size and override the default number of placement groups.

You can set these values when running pool commands. You can also override the defaults by adding new ones in the [global] section of the Ceph configuration file.



# By default, Ceph makes 3 replicas of objects. If you want to set 4
# copies of an object as the default value--a primary copy and three replica
# copies--reset the default values as shown in 'osd pool default size'.
# If you want to allow Ceph to write a lesser number of copies in a degraded
# state, set 'osd pool default min size' to a number less than the
# 'osd pool default size' value.

osd_pool_default_size = 4  # Write an object 4 times.
osd_pool_default_min_size = 1 # Allow writing one copy in a degraded state.

# Ensure you have a realistic number of placement groups. We recommend
# approximately 100 per OSD. E.g., total number of OSDs multiplied by 100
# divided by the number of replicas (i.e., osd pool default size). So for
# 10 OSDs and osd pool default size = 4, we'd recommend approximately
# (100 * 10) / 4 = 250.

osd_pool_default_pg_num = 250
osd_pool_default_pgp_num = 250

5.3. Additional Resources

  • See all the Red Hat Ceph Storage pool, placement group, and CRUSH configuration options in Appendix E for specific option descriptions and usage.