RE: Changing shared_buffers without restart

> all the possible scenarios. But now I'm reworking it along the lines suggested
> by Thomas, and will address those as well. Thanks!

Thanks for the info, Dmitry.
Just want to confirm my understanding of Thomas' suggestion and your discussions... I think the simpler and more
portablesolution goes something like the following?  

* For each BP resource segment (main, desc, buffers, etc):
    1. create an anonymous file as backing
    2. mmap a large reserved shared memory area with PROTO_READ/WRITE + MAP_NORESERVE using the anon fd
    3. use ftruncate to back the in-use region (and maybe posix_fallocate too to avoid SIGBUS on alloc failure during
first-touch),but no need to create a memory mapping for it 
    4. also no need to create a separate mapping for the reserved region (already covered by the mapping created in 2.)

|-- Memory mapping (MAP_NORESERVE) for BUFFER --|
|-- In-use region --|----- Reserved region -----|

* During resize, simply calculate the new size and call ftruncate on each segment to adjust memory accordingly, no need
tommap/munmap or modify any memory mapping. 

I tried this approach with a test program (with huge pages), and both expand and shrink seem to work as expected --for
shrink,the memory is freed right after the resize ftruncate. 

Regards,

Jack Ng