In this article, we will delve into the intricacies of PostgreSQL's architecture, shedding light on how its processes and memory management work.
Detailed Overview of Process Architecture
Postgres Server Process
The postgres server process, also known as the postmaster in earlier versions, acts as the parent process responsible for managing the entire database cluster. When started, it allocates shared memory, initiates background processes, and waits for client connections. Upon receiving a connection request, it spawns a backend process to handle queries issued by the connected client. Each PostgreSQL server listens on a specific network port (default is 5432) and can run multiple servers on the same host by using different port numbers.
Backend Processes
Backend processes, also referred to as "postgres" are started by the postgres server process. Each backend process is dedicated to handling queries from a connected client. It establishes a TCP connection with the client and remains active until the client disconnects. It's important to note that each backend process operates on a single database, and clients must explicitly specify the database when connecting to a PostgreSQL server. The maximum number of concurrent clients is controlled by the configuration parameter max_connections.
Background Processes
PostgreSQL employs various background processes to perform specific tasks for efficient database management. These processes include the background writer, checkpointer, autovacuum launcher, WAL writer, statistics collector, logging collector, and archiver. The background writer handles the writing of dirty pages to persistent storage, while the checkpointer performs checkpoint processes. The autovacuum launcher periodically invokes autovacuum-worker processes for vacuuming. The WAL writer flushes the Write-Ahead Log (WAL) data to disk, ensuring data durability. The statistics collector gathers statistics information, and the logging collector writes error messages to log files. The archiver executes archiving logging for data backup purposes.
Memory Architecture
Local Memory Area
Each backend process in PostgreSQL allocates a local memory area for query processing. This area is divided into several sub-areas, including work_mem and maintenance_work_mem. The work_mem is utilized for sorting tuples, joining tables, and performing operations like ORDER BY and DISTINCT. The maintenance_work_mem is used for maintenance operations such as VACUUM and REINDEX.
Shared Memory Area
PostgreSQL also utilizes a shared memory area that is allocated when the server starts up. This area is shared among all processes of the PostgreSQL server. The shared memory area includes sub-areas such as the shared buffer pool, which stores frequently accessed pages from tables and indexes. The WAL buffer is a buffering area for Write-Ahead Log data before it is written to disk. Additionally, there is a commit log (CLOG) for tracking transaction states.
Conclusion
Understanding the process and memory architecture of PostgreSQL is crucial for effectively managing and optimizing the performance of your PostgreSQL deployments. By grasping the roles and functionalities of the postgres server process, backend processes, and background processes, as well as the organization of local and shared memory, you gain insights into the inner workings of PostgreSQL. This knowledge empowers you to make informed decisions when configuring, monitoring, and troubleshooting your PostgreSQL databases.
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