Re: basic question (shared buffers vs. effective cache

On Mon, 10 May 2004, Jack Orenstein wrote:

> scott.marlowe wrote:
> >
> > shared_buffers is the amount of space postgresql can use as temp memory
> > space to put together result sets. It is not intended as a cache, and
> > once the last backend holding open a buffer space shuts down, the
> > information in that buffer is lost. If you're working on several large
> > data sets in a row, the buffer currently operates FIFO when dumping old
> > references to make room for the incoming data.
> >
> > Contrast this to the linux or BSD kernels, which cache everything they can
> > in the "spare" memory of the computer. This cache is maintained until
> > some other process requests enough memory to make the kernel give up some
> > of the otherwise unused memory, or something new pushes out something old.
>
> Do checkpoints operate on the Postgres-managed buffer, or the kernel-managed
> cache?

Checkpoints consist of writing the postgres managed data in the buffers to
the drive, which is cached by the kernel, then issuing an fsync to tell
the kernel to write it out to disk, so it affects both.