PG和PGP的区别是什么？调整 PGP 会不会引起 PG 内的对象的分裂？

首先来一段英文关于PG和PGP区别的解释：

PG = Placement Group
PGP = Placement Group for Placement purpose

pg_num = number of placement groups mapped to an OSD

When pg_num is increased for any pool, every PG of this pool splits into half, but they all remain mapped to their parent OSD.

Until this time, Ceph does not start rebalancing. Now, when you increase the pgp_num value for the same pool, PGs start to migrate from the parent to some other OSD, and cluster rebalancing starts. This is how PGP plays an important role.
By Karan Singh

以上是来自邮件列表的 Karan Singh 的PG和PGP的相关解释，他也是 Learning Ceph 和 Ceph Cookbook的作者，以上的解释没有问题，我们来看下具体在集群里面具体作用

• PG是指定存储池存储对象的目录有多少个，PGP是存储池PG的OSD分布组合个数
• PG的增加会引起PG内的数据进行分裂，分裂到相同的OSD上新生成的PG当中
• PGP的增加会引起部分PG的分布进行变化，但是不会引起PG内对象的变动