KB450109 – Placement Groups

Last modified: October 23, 2019
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Placement Group Sizing

Sizing placement group(PG) is very important. You can always increase PG size later but you can never decrease.

Keep in mind increasing the # of PGs will cause all data to begin a migration – this will put a strain on your cluster performance until it is complete.

It is best to follow:

  1. Size correctly first with pg_calculator. https://ceph.com/pgcalc/
  2. If you’re unsure what to choose for the correct # of PG, start with 64 PG per pool and increase the PG count at a later date. Ideally, before you put data on the pool.

Off hand rule of thumb is as follows:

  Less than 5 OSDs set pg_num to 128
  Between 5 and 10 OSDs set pg_num to 512
  Between 10 and 50 OSDs set pg_num to 1024
  If you have more than 50 OSDs, you need to understand the tradeoffs and how to calculate the pg_num value by yourself
  For calculating pg_num value by yourself please take help of pgcalc tool

If you’re using multiple device classes (ssd,hdd,nvme), treat each device class as if it was a separate cluster when it comes to PG calcs. (i.e. If the pool is going to live on the ssd device class – the # of OSDs would equal only the number of ssds in the cluster and the percentage of data would be 100%)

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