Scalable and High available Distributed File System Metadata Service Using gRPC, RocksDB and RAFT
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Alluxio Community Office Hour Apr 7, 2020 For more Alluxio events: https://www.alluxio.io/events/ Speaker: Bin Fan Alluxio (alluxio.io) is an open-source data orchestration system that provides a single namespace federating multiple external distributed storage systems. It is critical for Alluxio to be able to store and serve the metadata of all files and directories from all mounted external storage both at scale and at speed. This talk shares our design, implementation, and optimization of Alluxio metadata service (master node) to address the scalability challenges. Particularly, we will focus on how to apply and combine techniques including tiered metadata storage (based on off-heap KV store RocksDB), fine-grained file system inode tree locking scheme, embedded state-replicate machine (based on RAFT), exploration and performance tuning in the correct RPC frameworks (thrift vs gRPC) and etc. As a result of the combined above techniques, Alluxio 2.0 is able to store at least 1 billion files with a significantly reduced memory requirement, serving 3000 workers and 30000 clients concurrently. In this Office Hour, we will go over how to: - Metadata storage challenges - How to combine different open source technologies as building blocks - The design, implementation, and optimization of Alluxio metadata service
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Scalable and High available Distributed File System Metadata Service Using gRPC, RocksDB and RAFT
- 1. Building High-Performance, Concurrent & Scalable Filesystem Metadata Services Bin Fan @ Alluxio Online Office Hour Featuring gRPC, Raft, and RocksDB
- 2. ● PhD CS@CMU ● Founding Member, VP of Open Source @ Alluxio ● Email: binfan@alluxio.com About Me Bin Fan
- 3. Alluxio Overview • Open source data orchestration • Commonly used for data analytics such as OLAP on Hadoop • Deployed at Huya, Walmart, Tencent, and many others • Largest deployments of over 1000 nodes
- 5. Architecture
- 7. Alluxio Master • Responsible for storing and serving metadata in Alluxio • Data structure of the Filesystem Tree (namespace) ■ Can include mounts of other file system namespaces ■ The size of the tree can be very large! • Data structure to map files to blocks and their locations ■ Very dynamic in Alluxio • Who is the primary master ■ One primary + several standby masters
- 8. Challenges
- 9. Metadata Storage Challenges • Storing the raw metadata becomes a problem with a large number of files • On average, each file takes 1KB of on-heap storage • 1 billion files would take 1 TB of heap space! • A typical JVM runs with < 64GB of heap space • GC becomes a big problem when using larger heaps
- 10. Metadata Serving Challenges • Common file operations (ie. getStatus, create) need to be fast • On heap data structures excel in this case • Operations need to be optimized for high concurrency • Generally many readers and few writers for large-scale analytics • The metadata service also needs to sustain high load • A cluster of 100 machines can easily house over 5k concurrent clients! • Connection life cycles need to be managed well • Connection handshake is expensive • Holding an idle connection is also detrimental
- 11. Solving Scalable Metadata Storage Using RocksDB
- 12. Tiered Metadata Storage => 1 Billion Files 12 Alluxio Master Local Disk RocksDB (Embedded) ● Inode Tree ● Block Map ● Worker Block Locations On Heap ● Inode Cache ● Mount Table ● Locks
- 13. RocksDB • https://rocksdb.org/ • RocksDB is an embeddable persistent key-value store for fast storage
- 14. Working with RocksDB • Abstract the metadata storage layer • Redesign the data structure representation of the Filesystem Tree • Each inode is represented by a numerical ID • Edge table maps <ID,childname> to <ID of child> Ex: <1foo, 2> • Inode table maps <ID> to <Metadata blob of inode> Ex: <2, proto> • Two table solution provides good performance for common operations • One lookup for listing by using prefix scan • Path depth lookups for tree traversal • Constant number of inserts for updates/deletes/creates
- 15. Effects of the Inode Cache • Generally can store up to 10M inodes • Caching top levels of the Filesystem Tree greatly speeds up read performance • 20-50% performance loss when addressing a filesystem tree that does not mostly fit into memory • Writes can be buffered in the cache and are asynchronously flushed to RocksDB • No requirement for durability - that is handled by the journal
- 16. Self-Managed Quorum for Leader Election and Journal Fault Tolerance Using Raft
- 17. Built-in Fault Tolerance • Alluxio Masters are run as a quorum for journal fault tolerance • Metadata can be recovered, solving the durability problem • This was previously done utilizing an external fault tolerance storage • Alluxio Masters leverage the same quorum to elect a leader • Enables hot standbys for rapid recovery in case of single node failure
- 18. RAFT • https://raft.github.io/ • Raft is a consensus algorithm that is designed to be easy to understand. It's equivalent to Paxos in fault- tolerance and performance. • Implemented by https://github.com/atomix/copycat
- 19. • Consensus achieved internally • Leading masters commits state change • Benefits • Local disk for journal • Challenges • Performance tuning A New HA Mode with Self-managed Services 19 Standby Master Leading Master Standby Master Raft State Change State Change State Change
- 20. High-Performance and Unified RPC Framework Using gRPC
- 21. RPC System in Alluxio 1.x • Master RPC using Thrift • Filesystem metadata operations • Worker RPC using Netty • Data operations • Problems • Hard to maintain and extend two systems • Thrift is not maintained, no streaming RPC support Alluxio Master Alluxio Worker Application Alluxio Client Thrift RPC Thrift RPC Netty RPC
- 22. gRPC • https://grpc.io/ • gRPC is a modern open source high performance RPC framework that can run in any environment • Works well with Protobuf for serialization
- 23. Unified RPC Framework in Alluxio 2.0 • Unify all RPC interfaces using gRPC • Benefits • Streaming I/O • Protobuf everywhere • Well maintained & documented • Challenges • Performance tuning Alluxio Master Alluxio Worker Application Alluxio Client gRPC gRPC gRPC
- 24. gRPC Transport Layer • Connection multiplexing to reduce the number of connections from # of application threads to # of applications • Solves the connection life cycle management problem • Threading model enables the master to serve concurrent requests at scale • Solves the high load problem • High metadata throughput needs to be matched with efficient IO • Consolidated Thrift (Metadata) and Netty (IO) Check out this blog for more details: https://www.alluxio.com/blog/moving-from-apache-thrift-to-grpc- a-perspective-from-alluxio
- 25. Questions? Alluxio Website - https://www.alluxio.io Alluxio Community Slack Channel - https://www.alluxio.io/slack Alluxio Office Hours & Webinars - https://www.alluxio.io/events
- 26. 1 3 70 210 750 1080 Fast Growing Developer Community Started as Haoyuan Li’s PhD project “Tachyon” v1.0 Feb ‘16 v0.6 Mar ‘15 v0.2 Apr ‘13 v0.1 Dec ‘12 v2.1 Nov ‘19 v1.8 Jul ‘18 Open sourced in under Apache 2.0 License
- 27. Consumer Travel & TransportationTelco & Media TechnologyFinancial Services Retail & Entertainment Data & Analytics Services Deployed in Hundreds of Companies
- 28. Deployed at Scale in Different Environment On-Prem • Huya: 1300+ nodes • Sogou: 1000+ nodes • Momo: 850 nodes Single Cloud • Bazaarvoice: AWS • Ryte: AWS • Walmart Labs: GCP Hybrid Cloud • DBS Bank • ING Bank • Comcast
- 29. Metadata Storage Challenges • Durability for the metadata is important • Need to restore state after planned or unplanned restarts or machine loss • The speed at which the system can recover determines the amount of downtime suffered • Restoring a 1TB sized snapshot takes a nontrivial amount of time!
- 30. Example RocksDB Operations • To create a file, /s3/data/june.txt: • Look up <rootID, s3> in the edge table to get <s3ID> • Look up <s3ID, data> in the edge table to get <dataID> • Look up <dataID> in the inode table to get <dataID metadata> • Update <dataID, dataID metadata> in the inode table • Put <june.txtID, june.txt metadata> in the inode table • Put <dataId, june.txt> in the edge table • To list children of /: • Prefix lookup of <rootId> in the edge table to get all <childID>s • Look up each <childID> in the inode table to get <child metadata>
- 31. Tiered Metadata Storage • Uses an embedded RocksDB to store inode tree • Solves the storage heap space problem • Developed new data structures to optimize for storage in RocksDB • Internal cache used to mitigate on-disk RocksDB performance • Solves the serving latency problem • Performance is comparable to previous on-heap implementation • [In-Progress] Use tiered recovery to incrementally make the namespace available on cold start • Solves the recovery problem
- 32. Solutions: Combining Different Open- Source Technologies as Building Blocks
- 33. • Running Alluxio in HA • Zookeeper: Serve and elect the leader master for HA • HDFS: Journal Storage shared among masters • Problems • Limited choice of journal storage • local, streaming writes • Hard to debug/recover on service outrage • Hard to maintain Alluxio 1.x HA Relies on ZK/HDFS 33 Standby Master Leading Master Standby Master Shared Storage write journal Hello, leader read journal