Re: BUG #18049: dynamic_shared_memory_type's value `posix` doesn't have any effect, syscall shm_get executes

The following bug has been logged on the website:

Bug reference:      18049
Logged by:          Jitender Kumar
Email address:      jkr0103@gmail.com
PostgreSQL version: 12.1
Operating system:   ubuntu 20.04
Description:        

Expected `shm_open` syscall for the `posix` value of
`dynamic_shared_memory_type` parameter but strace shows `shm_get` being
executed.

src/backend/port/sysv_shmem. says:
 * As of PostgreSQL 9.3, we normally allocate only a very small amount of
 * System V shared memory, and only for the purposes of providing an
 * interlock to protect the data directory.

while src/backend/storage/ipc/dsm_impl.c says:

 * This file provides low-level APIs for creating and destroying shared
 * memory segments using several different possible techniques.  We refer
 * to these segments as dynamic because they can be created, altered, and
 * destroyed at any point during the server life cycle.  This is unlike
 * the main shared memory segment, of which there is always exactly one
 * and which is always mapped at a fixed address in every PostgreSQL
 * background process.

So AIUI you should always see a call like this when Postgres starts.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com