Ceph: Designing and Implementing Scalable Storage Systems

Promotions

The next tuning options that should be looked at are the ones that define the HitSets and the required recency to trigger a promotion:

    hitset_count    hitset_period

The hitset_count setting controls how many HitSets can exist before the oldest one starts getting trimmed. The hitset_period setting controls how often a HitSet should be created. If you are testing tiering in a laboratory environment, it should be noted that I/O to the PG needs to be occurring in order for a HitSet to be created; on an idle cluster, no HitSets will be created or trimmed.

Having the correct number and controlling how often HitSets are created is a key to being able to reliably control when objects get promoted. Remember that HitSets only contain data ...