CentOS 7 / Shared Memory - PostgreSQL (Confused).

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I have a fresh install of CentOS7 (x64) setup within HyperV (Server 2008 R2), I've allocated it a 80GB HDD and 8GB RAM.  I've also installed PostgreSQL 9.5 for use with my db (currently it's running on Server 2008 R2/PostgreSQL 9.0), which I am now looking to tweak the performance for.  So the first thing I am looking to tweak is the amount of shared memory available to the db by allocating 4GB to Shared Memory (shmmax), but if I run ipcs -lm I get;

------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 18014398509465599
max total shared memory (kbytes) = 18014398442373116
min seg size (bytes) = 1


This would seem to indicate I have a far larger swap file than should be possible.  Does anyone know why this is occurring, is it a bug within my VM setup (I'm seeing it in other CentOS VM's I have setup)?


Given the above should I skip doing anything with shmmax and move on to adjusting other settings within PostgreSQL.conf and carry on testing with pgbench?



*note* I would consider myself a novice with Linux, I know some command line and a little around it's workings but it's still very much a learning process for me at the moment.  Really starting to see how much of a sheltered life I've lived running Windows Servers.

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Get your config sorted first, then deal with the VM settings. Hyper-V has always been really fussy in my experience, though; so I'm not really a good one to specifically answer why it isn't changing the limit for you.

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