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Learning Ceph - Second Edition by Karan Singh, Vaibhav Bhembre, Anthony D'Atri

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Placement Group calculations

Choosing the correct number of PGs for each pool is one of the most important decisions when planning a Ceph cluster. A value that is too high or too low can severely affect performance. Too few PGs will result in uneven data distribution and excessive data movement during backfill/recovery. Too many PGs increase the overhead the cluster incurs when ensuring data availability, including memory usage by OSD daemons.

We can target the total number of placement groups for a pool using the following formula:

Put another way the ratio can be calculated:

PGPerOSD = (Total PGs) / (Total OSDs / Replication factor)

Choosing ...

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