Question on memory management sysv/posix on Linux

Hi

I have a question about memory management in PostgreSQL. I understand the default on Linux usually is "posix" which creates files in /dev/shm. With the default settings of 128MB for shared_buffers this is the result:

ls -la /dev/shm
total 4
drwxrwxrwt 2 root root 60 Feb 17 11:19 .
drwxr-xr-x 18 root root 3100 Feb 17 09:36 ..
-rw------- 1 postgres postgres 2316 Feb 17 11:19 PostgreSQL.1804289383

Question: Why is this file only 2316 bytes?

Switching to sysv (same setting for shared_buffers):

ipcs -m

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x0052e2c1 622592 postgres 600 56 5
0x6b8b4567 655361 postgres 600 2316 5

Again 2316 bytes. Why?

Is the 56 bytes segment related to Robert's post and only used for determining which processes are attached?
http://rhaas.blogspot.com/2012/06/absurd-shared-memory-limits.html

Thanks in advance
Daniel