ceph/doc/rados/configuration/pool-pg.conf

[global]

	# By default, Ceph makes 3 replicas of RADOS objects. If you want to maintain four
	# copies of an object 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 accept an I/O operation to a degraded PG,
	# set 'osd_pool_default_min_size' to a number less than the
	# 'osd_pool_default_size' value.

	osd_pool_default_size = 3  # Write an object 3 times.
	osd_pool_default_min_size = 2 # Accept an I/O operation to a PG that has two copies of an object.

	# 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.
        # always use the nearest power of 2

	osd_pool_default_pg_num = 256
	osd_pool_default_pgp_num = 256