Ceph: Designing and Implementing Scalable Storage Systems

Hammer – CRUSH_V4

The Hammer tunable profile does not affect the mapping of existing CRUSH maps simply by changing the profile and requires manual manipulation of the CRUSH map by enabling a new straw bucket type on CRUSH buckets:

• straw2: The straw2 bucket resolves several initial limitations of the original straw bucket algorithm. The major change is that with the initial straw buckets, changing the weight of a bucket item would lead to multiple PG mapping changes of other bucket items outside the item that was actually reweighted. straw2 will allow only changing mappings to or from the bucket that was actually reweighted:
  • Migration impact: Changing a bucket type from straw to straw2 will result in a fairly small amount of data movement, ...