![]() ![]() During a COMMIT or earlier the WAL Writer process reads them and appends them to the end of the current WAL file. The synchronization from RAM to disk consists of two steps.įirst, whenever the content of a page changes, a WAL record is created containing the delta-information (difference between the old and new content) and stored in another area of Shared Memory. They ensure that the cache is - after a short time delay - in sync with files. The Background Writer processes and the Checkpointer takes care of that. Modified pages are called dirty pages (or dirty buffers) and before they can be evicted they must be written back to disk. As long as the content of such pages hasn't changed, this is not a problem. Shared Memory is limited in size and it can become necessary to evict pages. Read accesses affect only the cache whereas write accesses are accomplished by writing to a log, the so-called write-ahead-log or WAL. Accesses to such caches are much faster than accesses to disk. Instead, they are done in a cache in the Shared Memory that mirrors the file pages. Such I/O-activities are not done directly on the disks. This is carried out by the client's Backend process. The next figure visualizes the main aspects of RAM, processes, files, and their collaboration.Ĭlient requests like SELECT or UPDATE usually lead to the necessity to read or write data. In response to new client connection attempts he launches new Backend processes and delegates authentication, communication, and the handling of all further requests to them. Subsequently, the Postmaster listens to its configured system port. It loads configuration files, allocates Shared Memory, and starts the other collaborating processes of the Instance: Background Writer, Checkpointer, WAL Writer, WAL Archiver, Autovacuum, Statistics Collector, Logger, and more. PostgreSQL does not use threading.Īt startup time, the Instance is launched by a single process denoted the Postmaster. The Instance is a group of server-side processes acting on the common Shared Memory. This leads to other activities (file access, WAL, vacuum. Such a Backend process handles the client's requests by acting on the Shared Memory. The server launches a single process for each client connection. Instead, they send requests to the Server and receive the requested data from there. Each Client process connects to one Backend process at the server site.Ĭlients do not have direct access to database files and the data stored in them. PostgreSQL implements a client-server architecture. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |