Postgresql不直接在硬盘上修改数据，它先把数据读到Postgresql共享内存缓冲区中，Postgresql后端在共享内存缓冲区中对数据读/写后，再把它们放回硬盘。 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. Postgresql的默认设置分配了64个共享缓冲区，每个缓冲区8k，增加缓冲区的数目使数据库后端更有可能在缓存中找到它们需要的信息，这避免了使用代价更高的系统请求。可以通过修改postmaster命令的一个参数或者postgresql.conf配置文件中的shared_buffers这一项来更改共享缓冲区数目。