POSTGRESQL Shared Buffer Cache

POSTGRESQL does not directly change information on disk. Instead, it requests data be read into the POSTGRESQL shared buffer cache. POSTGRESQL backends then read/write these blocks, and finally flush them back to disk.

Backends that need to access tables first look for needed blocks in this cache. If they are already there, they can continue processing right away. If not, an operating system request is made to load the blocks. The blocks are loaded either from the kernel disk buffer cache, or from disk. These can be expensive operations.

The default POSTGRESQL configuration allocates 64 shared buffers. Each buffer is 8 kilobytes. Increasing the number of buffers makes it more likely backends will find the information they need in the cache, thus avoiding an expensive operating system request. The change can be made with a postmaster command-line flag or by changing the value of shared_buffers in postgresql.conf.