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

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OSD variance and fillage

The variance in OSD utilization is crucial in determining whether one or more OSDs need to be reweighted to even out workload. A reasonably uniform distribution of PGs, and thus of data, among OSDs is also important so that outliers allocated significantly more than their share do not become full. In this section we'll explore this phenomenon and how to watch for it. In Chapter 6, Operations and Maintenance, we described a strategy for mitigating uneven distribution; here we explore why it happens and how adjust thresholds to meet your local needs.

The variance for some OSDs can increase when objects are not allocated in a balanced manner across all PGs. The CRUSH algorithm is designed to treat every PG as equal and ...

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