Greetings. I have a ceph cluster deployed on 3 nodes. Each node has 4 OSDs. There are 12 OSDs in total.

Initially, 32 PGs were created. Autoscale PG is off. Recently ceph started warning that the recommended amount of PG is being used. I increased the number of PGs through the interface to the recommended 64.

However, I was interested in calculating the number of PGs and it is not entirely clear how many should be set. Using a calculator and recommendations, I calculated using the formula: 12 OSD * 100/3 replicas = 400, rounded up to 512. However, I did not dare to set such a value as the recommendations differ. For example, the calculator on the website

https://old.ceph.com/pgcalc/ gives 256. Now I set 128. So what is the correct value? Also in the recommendations it is written that the number of PGPs should be equal to PG. However, with increasing the number of PGs from 64 to 128 from the interface, the number of PGPs increased only to 77. I saw this using the commands:

```
ceph osd pool get ceph pgp_num
ceph osd pool get ceph pg_num
```

Is it correct to manually increase PGP to 128 with the command:

`ceph osd pool set ceph pgp_num 128`

I also have one more pool,

`device_health_metrics`

What is its purpose?

UPD: Number of PGP change automatically to 128 after rebalancing