As has been covered earlier, promotions are very expensive operations in tiering and care should be taken to make sure that they only happen when necessary. A large part of this is done by carefully tuning the HitSet and recency settings. However, in order to limit the impact of promotions, there is an additional throttle that restricts the number of promotions to a certain speed. This limit can either be specified as number of bytes or objects per second via two OSD configuration options:
osd_tier_promote_max_bytes_sec
osd_tier_promote_max_objects_sec
The default limits are 4 MBps or five objects a second. While these figures may sound low, especially when compared with the performance of the latest SSDs, their primary goal is to minimize the impact of promotions on latency. Careful tuning should be done to find a good balance on your cluster. It should be noted that this value is configured...
