how to validate an VM is allocated much memory than it need and set an appropriate memory size?

Question

I want to check if one VM is allocated much memory than it needs. If so, want like to recycle some free memory back. how to validate it ? From vmware client,if one VM's guest physical memory usage of one VM is not high and no swap happened, is that meaning we can refer to the ative memory to adjust memory that VM needed ?

and one more question about how to check one VM actually memory details(used,free) ? I ask as sometimes host memory is lower than VM memory,but from system, the total size is VM memory size, and used size is larger than active memory from vsphere client.

eg: one VM

from vsphere client

VM memory 32768MB, host memory 28343, guest memory%  13%.

from system(linux)

[root@ ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:            31G         12G        956M        7.7M         18G         18G
Swap:          1.9G        103M        1.8G
[root@ ~]# 

Answer 1

If you see you "buff/cache" is 18GB is more than 50% of you real memory. Normally this value is the 15% of your memory to adjunt or 20% depent of yours applications.

If your machine in any moment needs more first will use a "buff/cache"

Total Memory --> 31GB Real Used --> 12GB 15 % of total Memory for cache --> 5GB Real Used + cache memory = 12GB + 5GB

Consumed Host Memory - Amount of machine memory used on the host. Consumed memory includes Includes memory used by the Service Console, the VMkernel, vSphere services, plus the total consumed metrics for all running virtual machines.

Active Guest Memory - Amount of guest “physical” memory actively used.

I think that you only need 17GB!! If you do not want to adjust so much you can put 20GB

If you want be more acurate in your decision. I recommend use vmstat command every minute for a week or one day with hight system activity.

This article can help you: https://www.tecmint.com/linux-performance-monitoring-with-vmstat-and-iostat-commands/