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Hi,

Literally copying from Ceph documentation: http://docs.ceph.com/docs/master/rados/operations/placement-groups/#choosing-the-number-of-placement-groups

For instance a cluster of 10 pools each with 512 placement groups on ten OSDs is a total of 5,120 placement groups spread over ten OSDs, that is 512 placement groups per OSD. That does not use too many resources. However, if 1,000 pools were created with 512 placement groups each, the OSDs will handle ~50,000 placement groups each and it would require significantly more resources and time for peering.

So, answering your question, yes, it will "downgrade" your cluster performance in order to peer these PGs over your current OSD. When you add a new OSD, PGs need to be peering again through cluster OSDs consuming more resources that being with less PG.

Regards