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Major changes and new concepts: Simplify Repository to a value-store ------------------------------------ Move the responsibility of tracking relation metadata, like which relations exist and what are their sizes, from Repository to a new module, pgdatadir_mapping.rs. The interface to Repository is now a simple key-value PUT/GET operations. It's still not any old key-value store though. A Repository is still responsible from handling branching, and every GET operation comes with an LSN. Key --- The key to the Repository key-value store is a Key struct, which consists of a few integer fields. It's wide enough to store a full RelFileNode, fork and block number, and to distinguish those from metadata keys. See pgdatadir_mapping.rs for how relation blocks and metadata keys are mapped to the Key struct. Store arbitrary key-ranges in the layer files --------------------------------------------- The concept of a "segment" is gone. Each layer file can store an arbitrary range of Keys. TODO: - Deleting keys, to reclaim space. This isn't visible to Postgres, dropping or truncating a relation works as you would expect if you look at it from the compute node. If you drop a relation, for example, the relation is removed from the metadata entry, so that it appears to be gone. However, the layered repository implementation never reclaims the storage. - Tracking "logical database size", for disk space quotas. That ought to be reimplemented now in pgdatadir_mapping.rs, or perhaps in walingest.rs. - LSM compaction. The logic for checkpointing and creating image layers is very dumb. AFAIK the *read* code could deal with a full-fledged LSM tree now consisting of the delta and image layers. But there's no code to take a bunch of delta layers and compact them, and the heuristics for when to create image layers is pretty dumb. - The code to track the layers is inefficient. All layers are just stored in a vector, and whenever we need to find a layer, we do a linear search in it.
This module contains utilities for working with PostgreSQL file formats. It's a collection of structs that are auto-generated from the PostgreSQL header files using bindgen, and Rust functions to read and manipulate them. There are also a bunch of constants in `pg_constants.rs` that are copied from various PostgreSQL headers, rather than auto-generated. They mostly should be auto-generated too, but that's a TODO. The PostgreSQL on-disk file format is not portable across different CPU architectures and operating systems. It is also subject to change in each major PostgreSQL version. Currently, this module is based on PostgreSQL v14, but in the future we will probably need a separate copy for each PostgreSQL version. TODO: Currently, there is also some code that deals with WAL records in pageserver/src/waldecoder.rs. That should be moved into this module. The rest of the codebase should not have intimate knowledge of PostgreSQL file formats or WAL layout, that knowledge should be encapsulated in this module.