[ceph-users] how can i config pg_num

xiang.dai at iluvatar.ai xiang.dai at iluvatar.ai
Tue Oct 16 01:04:23 PDT 2018

I install a ceph cluster with 8 osds, 3 pools and 1 replication(as osd_pool_default_size) in 2 machines. 

I follow formula in http://docs.ceph.com/docs/mimic/rados/operations/placement-groups/#choosing-the-number-of-placement-groups to count pg_nu. 
Then get total pg_num equal to 192, i set each pool as 64. 

I get below warn: 
$ ceph -s 
cluster: id: fd64d9e4-e33b-4e1c-927c-c0bb56d072cf 
health: HEALTH_WARN too few PGs per OSD (24 < min 30 ) 

Then i change osd_pool_size to 2, warning miss which makes me confused. 

I read docs again, i have below questions: 

1.Between 5 and 10 OSDs set pg_num to 512 in doc, this pg_num is total pg num? 
If so, for 2 replications, the pg per osd is too low. 
If not, it means pg num per pool, for more pools, the pg per osd is too high. 

2.How count the min is 30? 

3.Why only change replication, the warning miss, seems that not count by the formula. 

4.The formula does not consider pool num, just consider replication and osd num. 
So for more pool, the formula need to divide pool num too, right? 

5.In http://docs.ceph.com/docs/mimic/rados/configuration/pool-pg-config-ref/, it says set 250 as default. 
This num is not power of 2, why set it? Is it right? 

If i set osd_pool_default_size as 2, does it mean need to set osd_pool_default_min_size as 1? 
If so, when osd_pool_default_size is 1, osd_pool_default_min_size equal to Zero? 
If not, for 2 machine: 
1) set osd_pool_default_size as 2 is meaningless, but it can solve ceph status warning. 
2) set osd_pool_default_size and osd_pool_default_min_size both 1? 

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