Osd - tiering - new cache modes


Add read-forward and read-proxy cache modes


  • Sage Weil (Inktank)

Interested Parties

  • Name (Affiliation)
  • Name (Affiliation)
  • Name

Current Status

The current forward cache mode will process the request locally if it exists in the tier and forward anything else.

Detailed Description

There are a couple other modes that make sense:
  • read-forward: if the op is a read and we miss in the cache tier, forward the request.
    • this is useful if the base tier is similar fast for reads to the cache tier but we have asymetric write performance or endurance
  • read-proxy: proxy the read on miss
    • like read-forward, do not promote when there is a miss. but, instead of replying with a redirect and making the client do the read, just do the read on their behalf. this improves latency for certain configurations.

Work items

Coding tasks

  1. osd: add read-forward mode
  2. osd: add read-proxy mode
  3. mon: add new modes to cachemode command set
  4. qa: add tests to the rados suite to exercise the different cache modes

Documentation tasks

  1. doc: add docs that describe new (and old!) modes and their behaviors, along with examples of when they would be useful