Ceph

h1. Osd - tiering - new cache modes

h3. Summary

Add read-forward and read-proxy cache modes

h3. Owners

* Sage Weil (Inktank)

h3. Interested Parties

* Name (Affiliation)

* Name (Affiliation)

* Name

h3. Current Status

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

h3. 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.

h3. Work items

h4. Coding tasks

# osd: add read-forward mode

# osd: add read-proxy mode

# mon: add new modes to cachemode command set

# qa: add tests to the rados suite to exercise the different cache modes

h4. Documentation tasks

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