Rbd - ordered crash-consistent write-back caching extension » History » Revision 3
Revision 2 (Tushar Gohad, 12/02/2015 06:23 AM) → Revision 3/6 (Tushar Gohad, 12/03/2015 02:08 PM)
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. [1] * 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