Checkpoints

All modifications before a checkpoint must have been made to data chunks. Logs of changes committed after a checkpoint are also allowed to appear in a checkpoint. During recovery, it will choose the newest checkpoint instead of the longest one.

Concurrency Control

PolarDB uses parallel raft protocol to coordinate multiple data chunk servers. PolarRaft is a consensus protocol inherited from Raft but it allows out-of-order I/O completion tolerance capabilities of database.

Data Model

Hardware Acceleration

PolarDB uses RDMA to connect storage nodes and compute nodes. This removes the bottleneck for I/O performance.

Isolation Levels

PolarDB uses a novel disk outage consistency snapshot. It does not block user I/O operations during snapshot. When user triggers snapshot event, polarCtrl (the control plane of polar file system) will notify polarSwitch (the API that identifies page id, offset information from compute nodes and identify storage nodes in chunck servers) to take a snapshot. Chunk server will then take a snapshot before committing the request. After chunck servers receive snapshot tag of requests, they will take a snapshot before the request and copy block mapping meta information. Future requests will modify chunks in copy-on-write mechanism. After snapshot finishes, polarDB will stop adding snapshot tag to requests and will stop copy-on-write.

Logging

Query Interface

Storage Architecture

Storage Organization

Each chunk server has a write ahead log (WAL). Any modification to a chunk server will be appended in log before updating the chunk.

System Architecture

Within shared-disk, PolarDB has multiple data chunk servers which consist of chunks of data. Each chunk server has its own stand-alone non-volatile memory SSD disk. Compute nodes (database server) read and write to the disk via remote direct memory access (RDMA).