Ceph

h1. Rbd - crash-consistent ordered write-back caching extension

h3. Rationale

RBD cache currently supports object-based DRAM caching only, with no ordered write-back support.  Content cacheability is limited by the size of DRAM - proposal here is to extend librbd to support -

* A new librbd read cache to support *LBA-based* caching with DRAM/*non-volatile* storage backends

* An *ordered* *write-back* cache that maintains checkpoints internally (or is structured as a data journal), such that writes that get flushed back to the cluster are always *crash consistent*. Even if one were to lose the client cache entirely, the disk image is still holding a valid file system that looks like it is just a little bit stale [1].  Should have durability characteristics similar to async replication if done right.  

* External *caching plug-in* interface – kernel and usermode

This enhancement has been discussed at the 2015 Ceph hackathon and also at the Tokyo summit with the Ceph/RBD core.  

This proposal is based on discussions in [1], [2] with Sage and Josh.  Proof points for client-side caching in [2], [3] and external caching plugin example in Maciej’s CAS POC presentation at the Ceph Performance Weekly Meeting on Nov 25, 2015.

Tunable ordered Write-back caching option should provide boost for streaming, log storage, some VDI workloads and eventually consistent databases.  Where write-back is not suitable, a read-only (write-through) option should help improve access latencies - VDI and database workloads for example.

h3. Owners

* Anjaneya Chagam (Intel)

* Tushar Gohad (Intel)

* Stephen Blinick (Intel)

h3. Interested Parties

* Sage Weil, Josh Durgin

h3. Project Phases (current proposal)

* Phase1:  private, LBA-level, read-only cache - plumb in new generic caching layer in librbd for non-volatile storage-backend – possible ObjectCacher replacement.  Read-only cache, designed with scope for write-back caching and pluggability extensions in later phases

* Phase2: node-local, shared read-only cache. Private ordered/crash-consistent write-back cache

* Phase3: node-local, shared ordered/crash-consistent write-back cache.  External caching plugins – Intel CAS, dm-cache, etc

h3. References:

[1] http://ceph-users.ceph.narkive.com/0LDAkjYy/persistent-write-back-cache

[2] Ceph Hackathon’15 and Tokyo Summit Discussions on RBD write-back caching

[3] Whitepaper: database workloads using RBD with dm-cache and Ceph cache tiering